1637–1644 2026
This is an AI modernization of Descartes’ Philosophical Works (translated by John Veitch) into contemporary English. The original is available on Standard Ebooks.
Prefatory Note by the Author
If this Discourse seems too long to read in one sitting, it can be divided into six parts. In the first, you'll find various thoughts on the sciences. In the second, the key rules of the Method I've discovered. In the third, some rules for practical living that I've drawn from this Method. In the fourth, the arguments by which I establish the existence of God and the human soul — the foundations of my metaphysics. In the fifth, the scientific questions I've investigated, especially how the heart works and some other medical puzzles, as well as the difference between the human mind and that of animals. And in the last part, what I believe is needed to make greater progress in understanding nature than we've achieved so far, along with my reasons for writing this book.
Common sense is the most evenly distributed thing in the world. Everyone thinks they have plenty of it — even people who are impossible to satisfy in every other way don't usually wish they had more common sense than they do. And it's unlikely that everyone is wrong about this. What it really shows is that the ability to judge correctly and to tell truth from error — which is what we mean by "common sense" or "reason" — is naturally equal in all people. The reason we disagree with each other isn't that some of us are smarter than others. It's simply that we direct our thinking in different ways and don't focus on the same things. Having a powerful mind isn't enough on its own; what really matters is using it well. The sharpest minds are capable of the greatest achievements, but they're also capable of the worst mistakes. And people who move slowly but stay on the right road can get much further than those who sprint off in the wrong direction.
As for me, I've never thought my own mind was in any way better than average. In fact, I've often wished I could think as quickly as some people, or imagine things as vividly, or remember things as easily and completely. Beyond those abilities, I don't know of any other qualities that make a mind better. As for reason itself — the thing that makes us human and separates us from animals — I'm inclined to believe it's whole and complete in every person. On this point, I'll go along with the standard view among philosophers: that differences of more and less exist only in the incidental qualities of people, not in their fundamental nature.
That said, I'll freely admit that I've been incredibly lucky. Very early in life, I stumbled onto certain paths of thinking that led me to ideas and principles from which I've built a Method. This Method gives me a way — or so I believe — to gradually increase my knowledge and raise it, little by little, to the highest level that my modest abilities and limited lifespan will allow. I've already gotten such good results from it that, even though I tend to think pretty humbly of myself, and even though when I look at what most people spend their time doing, I can barely find anything that doesn't seem pointless, I still feel deeply satisfied with the progress I've made in the search for truth. I can't help looking forward to the future and believing that if there's any truly excellent and important pursuit in human life, it's the one I've chosen.
Of course, I could be wrong. Maybe what I'm taking for gold and diamonds is really just a bit of copper and glass. I know how easily we fool ourselves about our own achievements, and I also know how suspicious we should be of our friends' judgments when they're trying to be nice to us. But in this Discourse, I'm going to describe the paths I've followed and lay out my life like a painting, so that everyone can judge for themselves. That way, when I hear what people think about it, I'll have a new source of learning to add to the ones I already use.
So let me be clear about what I'm doing here. I'm not trying to teach anyone the method they should use to reason correctly. I'm only describing the way I've tried to guide my own thinking. People who set themselves up as teachers obviously think they know more than the people they're teaching — and if they get even the smallest thing wrong, they deserve the criticism they'll get. But this is just a story — a history, or if you prefer, a tale — in which you'll find some examples worth following alongside plenty of others you'd be wise to avoid. I hope it will be useful to some people without doing harm to anyone, and that my honesty will count for something.
Ever since I was a child, I loved books and learning. I was told that through study, a person could gain clear, certain knowledge of everything useful in life, and I wanted that desperately. But the moment I finished my entire course of studies — the point where you're officially admitted into the ranks of the educated — I completely changed my mind. I found myself tangled up in so many doubts and errors that the only thing I seemed to have learned was how much I didn't know. And I wasn't studying at some mediocre school. I was at one of the most famous institutions in all of Europe, where I figured that if there were learned people anywhere in the world, they'd be there. I had been taught everything the other students learned. On top of that, not content with just the official curriculum, I had read every book I could get my hands on about subjects considered the most fascinating and unusual. I knew what others thought of me, and I wasn't considered inferior to my classmates — some of whom were already being groomed to become professors themselves. Our era seemed to me as intellectually rich and full of brilliant minds as any before it. So I felt justified in judging everyone else's education by my own experience, and I concluded that there simply was no body of knowledge as reliable as I'd been led to believe.
Even so, I still respected what was taught in the schools. I recognized that learning languages is necessary for understanding ancient writers. That mythology stirs the imagination. That the great deeds recorded in history elevate the mind and, if read with good judgment, help sharpen it. That reading excellent books is like having a conversation with the finest minds of earlier centuries — a carefully curated conversation, where they share only their best ideas. That eloquence has tremendous power and beauty. That poetry has its own enchanting pleasures. That mathematics contains many brilliant discoveries that can satisfy our curiosity, advance technology, and reduce human labor. That books on ethics are full of useful advice and inspiring calls to virtue. That theology shows us the path to heaven. That philosophy gives us a way to talk about absolutely anything with an air of authority, impressing people who don't know better. That law, medicine, and the other sciences bring their practitioners honor and wealth. And finally, that it's worth spending some time on all of them — even the ones most riddled with superstition and error — so we can know what they're really worth and avoid being taken in.
But I felt I had already spent enough time on languages and on reading the ancients — their histories and their stories. Conversing with people from other eras is much like traveling: it's useful to learn something about the customs of different nations, so that we can judge our own more fairly and not assume that everything different from what we're used to is ridiculous and irrational — the way people tend to think when they've never left home. On the other hand, when you spend too much time traveling, you become a stranger in your own country. And people who are too fascinated by the past are usually out of touch with the present. What's more, fictional stories lead us to imagine things that are impossible, and even the most honest histories, if they don't outright misrepresent events or exaggerate their significance to make for better reading, at least leave out the most ordinary and unremarkable details. The result is that the picture they give us isn't quite true to life. People who try to model their behavior on such stories risk becoming like the knights-errant in old romances, dreaming up adventures that are beyond their abilities.
I had great admiration for eloquence and was enchanted by poetry, but I thought both were natural gifts rather than things you could learn from studying. The people who reason most clearly and organize their thoughts most effectively are always the most persuasive — even if they speak a rough dialect and know nothing about rhetoric. And the people with the most vivid imaginations, who can express their ideas with the most grace and charm, are the best poets — even if they've never studied the rules of poetry.
I was especially delighted by mathematics, because of its certainty and the clarity of its reasoning. But at the time, I didn't fully understand its true potential. I assumed mathematics was only useful for mechanical applications, and I was surprised that such strong, solid foundations had never been used to build anything more ambitious on top of them. On the other hand, I compared the writings of the ancient moral philosophers to elaborate, magnificent palaces built on nothing but sand and mud. They praise virtue to the skies and make it sound like the most valuable thing on earth, but they give us no reliable way to recognize it. And often what they dignify with such grand names turns out to be nothing more than emotional numbness, or arrogance, or despair, or cruelty.
I had great respect for theology and wanted as much as anyone to reach heaven. But I was given to understand, quite firmly, that the way to heaven is no less open to the most uneducated than to the most learned, and that the revealed truths that lead us there are beyond human understanding. I didn't dare subject them to the weakness of my own reasoning, and I thought that to properly examine them, a person would need some special help from above — something more than merely human.
As for philosophy, I'll just say this: I saw that it had been studied for centuries by the most brilliant minds, and yet there wasn't a single topic within it that wasn't still being argued about — nothing that was beyond doubt. I wasn't bold enough to think I'd do better than everyone else. And when I considered that on any given question, learned people could defend several conflicting opinions, even though only one of them could actually be true, I decided to treat anything that was merely probable as practically false.
As for the other sciences, since they all borrow their foundations from philosophy, I figured that nothing solid could be built on such shaky ground. And neither the prestige nor the money they offered was enough to motivate me to pursue them. I wasn't, thank God, in a position where I needed to sell my knowledge to make a living. And although I didn't make a show of scorning fame like some philosopher, I didn't think much of honors that could only be won through bogus credentials. As for the bogus sciences — I was confident I knew enough to avoid being fooled by the claims of an alchemist, the predictions of an astrologer, the tricks of a magician, or the boasting of anyone who pretends to know things they don't.
For all these reasons, as soon as I was old enough to leave my teachers behind, I gave up the study of books entirely. I resolved to seek no other knowledge than what I could find in myself, or in the great book of the world. I spent the rest of my youth traveling, visiting royal courts and armies, meeting people of all different temperaments and social positions, gathering all kinds of experiences, testing myself in whatever situations life threw my way, and above all, reflecting carefully on everything I encountered so I could actually learn from it. It seemed to me that I'd find much more truth in the everyday reasoning of ordinary people — people dealing with real situations where a bad judgment would immediately come back to bite them — than in the speculations of some scholar sitting in his study, thinking about abstract matters with no real consequences. After all, the further those speculations are from common sense, the more cleverness and ingenuity they require to seem plausible, which just flatters the scholar's ego all the more. And all along, I had one burning desire: to learn how to tell truth from falsehood, so I could see my way clearly through life and move forward with confidence.
It's true that while I was busy observing the customs of other people, I didn't find much solid ground for certainty there either. I noticed almost as much disagreement among different cultures as among the philosophers. But the greatest benefit I got from this experience was seeing that many things which seem completely absurd and ridiculous from our own perspective are widely accepted and approved by other great nations. This taught me not to believe too firmly in anything I'd been persuaded of only through example and habit. And in this way, I gradually freed myself from many errors that had been powerful enough to cloud my natural intelligence and keep me from truly listening to reason. But after I had spent several years studying the book of the world in this way, trying to gather experience wherever I could, I finally decided to turn inward and make myself the object of study. I resolved to use all the powers of my mind to figure out which paths I should follow — an undertaking that turned out far more successfully than it would have if I had never left my country or my books.
I was in Germany at the time, drawn there by the wars that were still going on. As I was making my way back to the army after the emperor's coronation, the onset of winter stranded me in a small town where I knew no one, had no distractions, and was — luckily — free from any pressing concerns or emotions. I spent entire days alone in a heated room, with nothing to do but think.
One of the very first thoughts that came to me was this: things designed by a single person tend to be more perfect than things cobbled together by many different hands. A building planned and built by one architect, for example, is usually more elegant and functional than one that several people have tried to patch together over the years, repurposing old walls for things they were never meant to do. Or think about cities: the ones that started as small villages and slowly grew into large towns are almost always badly laid out compared to cities that a single planner designed from scratch on open ground. Sure, some of the individual buildings in the old cities might be just as beautiful — or even more beautiful — than those in the planned ones. But when you look at how they're arranged, a big building here and a tiny one there, streets winding every which way, you'd think chance rather than human reason was behind the layout. And if you consider that there have always been officials whose job was to make sure private buildings looked good and contributed to the city overall, it's clear how hard it is to create something excellent when you're working with other people's materials.
Similarly, I figured that nations which started in a semi-civilized state and gradually built up their laws over time — shaped only by the experience of dealing with crimes and disputes as they came up — would end up with less perfect institutions than those communities that had followed a single wise lawmaker's plan from the very beginning. Along those lines, it's certain that the constitution of the true religion, whose laws come from God, must be incomparably better than any other. And to stick with human examples: I believe Sparta's greatness came not from the quality of each individual law (many of them were actually quite bizarre and even contrary to good morals) but from the fact that they were all designed by one person and all pointed toward a single purpose.
In the same way, I thought that the knowledge found in books — at least the kind built on probable reasoning rather than rigorous proof — since it's assembled from the opinions of many different people piled together, is further from the truth than the straightforward conclusions a sensible person draws from their own direct experience. And because every one of us had to pass through childhood before reaching adulthood, spending all that time governed by our desires and our various teachers (who often contradicted each other, and neither group always gave us the best advice), I concluded that it's nearly impossible for our judgments to be as sound and solid as they would have been if we'd had full use of our reason from the moment we were born and had always been guided by it alone.
Now, it's true that nobody tears down all the houses in a town just to rebuild them and make the streets prettier. But individual homeowners do sometimes demolish their own houses to build better ones, and sometimes they're forced to when the old ones are about to collapse from age or their foundations are crumbling. Using that as my model, I became convinced that it would be absurd for any private individual to try to reform a whole state by tearing it down and starting over. The same goes for trying to overhaul the entire body of the sciences, or the way they're taught in the universities.
But when it came to the opinions I myself had accepted up to that point, I decided I couldn't do better than to sweep them all away at once, so that I could later replace them — either with better ones, or perhaps even with the same ones after I'd checked them against reason. I was firmly convinced that this approach would let me lead my life far more successfully than if I just kept building on old foundations, relying on principles I'd taken on trust when I was young.
I did see various difficulties in this plan, but they weren't without solutions, and they were nothing compared to the difficulties involved in reforming public affairs. Large institutions, once toppled, are incredibly hard to set back up, or even to keep standing once they've been seriously shaken, and their collapse is always catastrophic. Besides, if there are imperfections in a state's constitution (and the sheer variety of constitutions out there is enough to assure us there are), custom has done a great deal to smooth over the rough edges, and has even managed to fix — quietly, over time — a number of problems that deliberate planning could never have handled as well. In short, the flaws are almost always more bearable than the upheaval required to fix them, just as mountain highways that wind and twist, once they've been worn smooth by heavy use, are much better to travel on than trying to take a straighter path by scrambling over rocks and down into ravines.
That's why I have no patience for those restless, meddling people who, despite having no authority or special ability to manage public affairs, are constantly dreaming up reforms. And if I thought this book of mine contained anything that might give the impression I was one of them, I would never have allowed it to be published. My ambitions have never gone beyond reforming my own opinions and building my knowledge on a foundation entirely my own. Even though I've been satisfied enough with the results to share this sketch of my approach, I'm absolutely not telling everyone else to do the same thing. Those whom God has given greater abilities may have even grander plans. But for most people, I'm genuinely worried that even this project of mine would be too much to safely imitate.
The decision to strip away all your past beliefs is not one everyone should make. Most people fall into one of two categories, and for neither is this a good idea. The first category includes people who are more confident in their own abilities than they should be: they rush to judgment and don't have the patience for careful, orderly thinking. If people like this give themselves permission to doubt their established opinions and leave the well-worn path, they'll never find the shortcut they're looking for. They'll just get lost and wander around for the rest of their lives. The second category includes people who have enough sense or humility to recognize that others are better at distinguishing truth from error. These people would do better to follow the opinions of those wiser individuals than to try to figure everything out on their own.
As for me, I would certainly have fallen into this second category if I'd only ever had one teacher, or if I'd never learned about the wild diversity of opinions that the greatest thinkers have held throughout history. But even back in school, I'd become aware that there is no opinion so absurd or unbelievable that some philosopher hasn't defended it. And later, during my travels, I noticed that people whose views are completely different from ours aren't barbarians or savages because of it — in fact, many of them use their reason just as well as we do, if not better. I also considered what a different person you become depending on where you grow up. Someone raised from infancy in France or Germany turns out quite differently from the same person raised in China or among indigenous peoples — and they had the exact same mind to begin with. Even in something as trivial as fashion: the style we loved ten years ago, and might love again ten years from now, looks ridiculous to us right now. All of this led me to conclude that our opinions are based far more on custom and example than on any real knowledge.
And one more thing: even where opinions are widely shared, majority agreement is no guarantee of truth when it comes to anything difficult to discover. In those cases, it's much more likely that one person will find the truth than that many will. But since I couldn't pick out anyone from the crowd whose opinions seemed worth following over all the rest, I found myself essentially forced to rely on my own reason to guide my life.
Like someone walking alone in the dark, I resolved to go very slowly and very carefully, so that even if I didn't make much progress, I would at least avoid falling. I didn't even rush to throw out all the opinions that had slipped into my beliefs without being put there by reason. Instead, I first took enough time to carefully plan out what I was doing, and to find the true method for arriving at knowledge of everything within my ability to understand.
In the past, I'd studied three subjects that seemed like they should be useful for this project: logic (from philosophy), and geometrical analysis and algebra (from mathematics). But when I examined them closely, I found problems with all three.
Logic: its syllogisms and most of its other rules are actually more useful for explaining what you already know — or even, like the method of Raymond Lull, for talking confidently about things you don't know — than for discovering new truths. And while logic does contain some genuinely correct and excellent principles, they're so mixed in with harmful or useless ones that separating the good from the bad is almost as hard as carving a Diana or Minerva out of a rough block of marble.
As for ancient geometrical analysis and modern algebra: both deal with highly abstract matters that seem to have no practical use. Geometry is so tied to shapes and figures that it can only exercise the mind at the cost of exhausting the imagination. And algebra is so bound by rigid rules and formulas that it becomes a confusing, obscure art that muddles the mind rather than cultivating it.
These considerations led me to look for a different method — one that would combine the strengths of all three disciplines while avoiding their weaknesses. And just as a state with few laws, strictly enforced, is often better governed than one with a huge legal code, I decided that instead of the enormous number of rules that make up logic, a small number of rules — strictly followed — would be perfectly sufficient.
I settled on four:
RULE ONE: Never accept anything as true unless I clearly know it to be so. That means carefully avoiding hasty judgment and preconceived ideas, and including nothing in my conclusions beyond what presents itself to my mind so clearly and distinctly that I have no reason to doubt it.
RULE TWO: Divide each problem I examine into as many parts as possible, breaking it down as far as needed for a proper solution.
RULE THREE: Direct my thinking in an orderly way, starting with the simplest and easiest things to understand, and climbing step by step toward knowledge of the more complex — even imposing an order on things that don't naturally come in any particular sequence.
RULE FOUR: In every case, make my lists so complete and my reviews so thorough that I can be sure nothing has been left out.
Those long chains of simple, easy reasoning that mathematicians use to reach their most difficult proofs had convinced me of something: that everything human beings are capable of knowing is connected in the same way, and that there is nothing so remote that we can't reach it or so hidden that we can't uncover it — as long as we avoid accepting anything false as true and always maintain the proper order of reasoning from one truth to the next.
I had no trouble figuring out where to start: with the simplest and easiest things to know. I was already aware that of all the people who have ever searched for truth in the sciences, only mathematicians have managed to produce actual demonstrations — meaning certain and undeniable proofs. So that was obviously the right starting point. I decided to begin by examining the simplest mathematical objects. I wasn't hoping to learn anything from this beyond getting my mind accustomed to nourishing itself on truth and refusing to accept unsound reasoning.
But I had no intention of trying to master every branch of what's usually called mathematics. Instead, I noticed that despite their different subjects, all mathematical sciences agree on one thing: they study the various relationships and proportions among their objects. So I thought it would be best to study these proportions in the most general way possible, without tying them to any particular objects, except where doing so would make understanding easier — while keeping it clear that my findings could be applied to anything else these proportions fit.
Then I realized that in order to understand these relationships, I'd sometimes need to consider them one by one and other times hold several in mind at once. For examining them individually, I decided to represent them as relationships between straight lines, since I couldn't think of anything simpler or easier to picture clearly in my mind. And for keeping track of many relationships at once, I'd express them using the shortest possible notation. In this way, I believed I could take the best of both geometrical analysis and algebra and use each one to fix the other's shortcomings.
And in fact, carefully following these few rules gave me — if I may say so — such facility in untangling every problem in these two sciences that in the two or three months I spent working on them, I not only solved problems I'd previously considered extremely difficult, but I also found that even for problems I couldn't yet solve, I could determine the method by which they could be solved and how far a solution was possible. This was all because I started with the simplest and most general truths, so that each truth I discovered became a tool for discovering the next one.
I hope this doesn't sound too boastful. Consider: when it comes to any particular truth, there is only one correct answer, and whoever finds it knows everything there is to know about that point. A child who has learned basic arithmetic and correctly adds up a set of numbers can be confident that they've found the right answer — the very same answer that lies within the reach of the greatest genius. In the end, the method that teaches you to follow the correct order and to make a complete accounting of all the conditions of the problem contains everything that gives the rules of arithmetic their certainty.
But what satisfied me most about this Method was the confidence it gave me that I was using my reason in all things, if not perfectly, at least as well as I possibly could. Beyond that, I could feel my mind gradually becoming accustomed to conceiving its objects more clearly and distinctly. And since I hadn't restricted this Method to any particular subject, I hoped to apply it to the problems of other sciences just as successfully as I had to algebra.
That said, I wasn't about to rush headlong into examining every difficulty in every science at once — that would have violated the very order my Method prescribed. Instead, I noticed that knowledge in all these fields depends on principles borrowed from philosophy, and in philosophy I had found nothing certain. So I decided I needed to establish those principles first.
But I also recognized that this was the most important investigation of all, and the one where hasty, premature judgment would be most dangerous. So I decided I shouldn't attempt it until I'd reached a more mature age (I was only twenty-three at the time). First, I needed to spend a good deal of time preparing: rooting out all the false opinions I'd accepted up to that point, gathering a wide range of experiences to give my reasoning something to work with, and continually practicing my chosen Method so I could get better and better at applying it.
Now, it's not enough — before you start rebuilding the house you live in — just to tear it down, gather materials and hire builders, or roll up your sleeves and get to work following a carefully drawn-up plan. You also need somewhere to stay while the construction is going on. In the same way, I didn't want to be paralyzed by indecision in my day-to-day life just because my reason was forcing me to suspend judgment on everything I believed. And I certainly didn't want to stop myself from living as happily as possible in the meantime. So I put together a provisional moral code — a set of three or four rules to live by — and I'd like to share them with you.
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**The first rule** was to obey the laws and customs of my country, to hold firmly to the faith in which, by the grace of God, I had been raised since childhood, and to guide my behavior in all other matters by the most moderate opinions — the ones farthest from any extreme — that were generally accepted in practice by the most sensible people around me.
Here was my reasoning: since I had already decided to treat all my own opinions as worthless (because I wanted to examine every single one of them), the best I could do in the meantime was to follow the opinions of the most sensible people. And sure, there are probably people among the Persians and the Chinese who are just as sensible as anyone in Europe. But practically speaking, it made sense to follow the opinions of the people I actually had to live with.
To figure out what those people really believed, I decided I should pay attention to what they *did* rather than what they *said*. There were two reasons for this. First, in our corrupt times, few people are willing to say exactly what they believe. Second — and this is more interesting — many people don't even *know* what they really believe. The mental act of believing something is different from the act of knowing that you believe it, and you often get one without the other.
Also, whenever I found opinions that seemed equally reasonable, I always chose the most moderate one. Partly because moderate views are the most practical, and probably the closest to the truth (since extremes are generally harmful). But also because if I happened to be wrong, I'd at least be closer to the right answer than if I'd picked one extreme and it turned out I should have picked the other.
I also classified as "extreme" any promise that would restrict my freedom. Not that I disapproved of laws that use vows and contracts to keep people committed to a good course of action — people with weak willpower sometimes need that kind of structure. And contracts are obviously necessary for business, even when the subject matter is relatively unimportant. But here's the thing: I couldn't find anything on earth that was completely immune to change. And since I particularly hoped to gradually *improve* my judgments rather than let them get worse, it would have been a serious offense against good sense to bind myself permanently to something just because I approved of it at one particular moment. What if it stopped being good later? Or what if I simply changed my mind for good reason?
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**My second rule** was to be as firm and decisive in my actions as I could possibly be, and once I'd committed to a course of action — even a doubtful one — to stick to it just as firmly as if I were completely certain about it.
Think of it like this: travelers who get lost in a forest shouldn't wander back and forth, and they definitely shouldn't just sit down in one spot. What they should do is pick a direction and walk in as straight a line as they can, without changing course for trivial reasons — even if it was pure chance that made them choose that direction in the first place. Because even if they don't end up exactly where they wanted to go, they'll at least reach the edge of the forest eventually, which is almost certainly better than staying stuck in the middle of it.
In the same way, real life often requires action and won't wait around while you figure out the perfect answer. So when we can't determine what's actually true, we should act on whatever seems most probable. And even if no option seems more likely than another, we still need to pick one — and then, for all practical purposes, treat that choice as if it were obviously true and certain. After all, the reasoning behind the decision was itself sound: it was the best we could do under the circumstances.
This principle was enough to free me from all the regret and guilt that typically torment indecisive people — the kind of people who, lacking any clear principle for making choices, decide one day that a certain course of action is best, only to abandon it the next day as the worst.
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**My third rule** was to always try to master myself rather than fortune, and to change my desires rather than the order of the world. More broadly, I wanted to train myself to accept that nothing is truly in our power except our own thoughts. That way, once I'd done my best regarding anything outside myself, whatever I still failed to achieve I could consider — as far as I was concerned — absolutely impossible.
This single principle seemed like enough to keep me from wanting things I couldn't have, and to keep me content. Here's why: our will naturally goes after only those things that our understanding presents as somehow attainable. So if we regard all external goods as equally beyond our control, we won't feel any more regret about lacking things that seem owed to us by birth (when we've been deprived of them through no fault of our own) than we feel about not owning the kingdoms of China or Mexico. Making a virtue of necessity, as the saying goes, we'll no more wish for health when sick, or freedom when imprisoned, than we currently wish for bodies as indestructible as diamonds or wings to fly with.
I'll admit, though: it takes long practice and frequent reflection to train yourself to see everything this way. And I believe this was essentially the secret behind the power of those ancient philosophers who managed to rise above the influence of fortune and, even in the midst of suffering and poverty, enjoy a happiness that their gods might have envied. By constantly reflecting on the limits that nature had set on their power, they became so thoroughly convinced that nothing was at their disposal except their own thoughts that this conviction alone was enough to prevent them from wanting anything else. And they gained such absolute mastery over their thoughts that they had real grounds for considering themselves richer and more powerful, freer and happier, than other people who — no matter how many gifts nature and fortune had showered on them — lacked this philosophy and could therefore never fully control what they wanted.
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Finally, to round out this moral code, I considered the various occupations people pursue in life, hoping to pick the best one. Without wanting to pass judgment on what anyone else does, I can say that I was convinced the best thing for me was to keep doing exactly what I was already doing: devoting my whole life to cultivating my reason and making as much progress as I could in the knowledge of truth, following the principles of the Method I'd set for myself.
Since I'd started applying this Method, it had given me such deep satisfaction that I couldn't imagine any way of life more fulfilling or more innocent. Every day, it helped me discover truths that seemed genuinely important and that most people were unaware of, and the pleasure this gave me occupied my mind so completely that I was indifferent to everything else.
Besides, all three of my rules were really grounded in this same goal of continuing my self-education. Since God has given each of us a certain light of reason to tell truth from error, I couldn't have been content to simply rest on other people's opinions unless I'd already committed to using my own judgment to examine those opinions as soon as I was qualified to do so. And I couldn't have followed those opinions without worry if I'd thought that doing so would cost me the chance of finding something more accurate. And I couldn't have restrained my desires or remained content if I hadn't been on a path that I was confident would lead me to all the knowledge I was capable of acquiring, and to the greatest real good I could ever hope to achieve.
Since we pursue or avoid things only based on whether our understanding presents them as good or bad, all that's needed for right action is right judgment — and for the best action, the most accurate judgment. That means acquiring all the virtues and everything else that is truly valuable and within our reach. And the confidence that you're making this kind of progress can't help but make you happy.
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Having set up these rules and filed them away alongside the truths of faith (which have always held first place in my beliefs), I concluded that I was free to begin getting rid of the rest of my opinions.
Since I thought I'd be better able to do this by engaging with the world than by staying shut up in the room where I'd had these thoughts, I set out traveling again before winter was even over. For the next nine years, I did nothing but wander from place to place, wanting to be a spectator rather than an actor in the drama of life. In every situation, I made a point of reflecting on what could reasonably be doubted and what might be a source of error, and I gradually uprooted from my mind all the mistakes that had crept in over the years.
I wasn't imitating the skeptics in this — those people who doubt just for the sake of doubting and never aim at anything beyond uncertainty itself. On the contrary, my whole purpose was to find solid ground, to dig past the loose dirt and sand until I hit rock or clay. And I think I was pretty successful at this. Whenever I tried to expose the falseness or uncertainty of the ideas I examined — not with weak guesswork, but with clear and certain reasoning — I never found anything so doubtful that it didn't yield at least some reliable conclusion, even if that conclusion was merely that the matter in question contained nothing certain.
And just as when tearing down an old house you normally save the usable materials for the new building, in the process of demolishing my poorly-founded opinions I made all kinds of observations and gained a wealth of experience that I later put to use in establishing more certain beliefs.
I also kept practicing the Method. Beyond generally conducting all my thoughts according to its rules, I regularly set aside time specifically for applying it to mathematical problems, and sometimes to questions in other sciences too — though only after I'd detached those questions from the shaky principles of their fields and made them essentially mathematical in nature. (The many examples throughout this volume will make this clear.)
And so, without appearing to live any differently from people who have no occupation other than spending their lives pleasantly and innocently — people who make a point of keeping their pleasures free from vice, and who fill their leisure with worthwhile activities to keep boredom at bay — I was quietly pursuing my project, making more progress in the knowledge of truth than I probably would have made if I'd done nothing but read books or talk with scholars.
Still, those nine years went by before I'd reached any firm conclusions about the questions that scholars argue over, or started looking for philosophical principles more certain than the commonly accepted ones. And the example of many brilliant thinkers who had attempted this same inquiry throughout history, apparently without success, made me think it must be an incredibly difficult task. I might not have attempted it so soon if I hadn't heard rumors going around that I'd already finished it.
I'm not sure where this impression came from. If my conversations played any part in creating it, that was probably because I'd been more openly honest about my ignorance than most people who've studied a bit tend to be, and because I'd perhaps explained my reasons for doubting many things that others considered certain — not because I'd ever bragged about having a philosophical system of my own.
But since I'm the kind of person who hates being thought of as something I'm not, I felt I had to do everything I could to actually deserve this reputation. So exactly eight years ago, I decided to move away from every place where I might run into people I knew, and I settled in this country — Holland — where the long war has created such an orderly society that the armies seem to serve no purpose other than helping the inhabitants enjoy the blessings of peace more securely. Here, in the middle of a bustling population that is focused on its own business and more concerned with its own affairs than curious about other people's, I've been able to enjoy all the conveniences of the most crowded cities while living as quietly and privately as if I were in the most remote desert.
I'm not entirely sure it's appropriate to share my earliest meditations here, because they're so abstract and unusual that most people might not find them interesting. But since the only way to judge whether the foundations I've laid are solid enough is to examine them directly, I feel I have to discuss them.
I had already noted, as I mentioned earlier, that in practical life it's sometimes necessary to act on opinions we know are uncertain, treating them as if they were beyond doubt. But now I wanted to focus entirely on the search for truth. And for that purpose, I thought the exact opposite approach was needed: I should reject as absolutely false anything in which I could find the slightest reason for doubt. My goal was to see whether, after all that rejection, anything would remain that was completely beyond question.
So I began. Since our senses sometimes deceive us, I decided to suppose that nothing is really as our senses show it to us. Since even the smartest people sometimes make logical errors — even in simple geometry — I assumed I was just as prone to mistakes, and I rejected as false all the reasoning I had previously accepted as solid proof. And finally, since the very same thoughts we have while awake can also come to us in our dreams — when none of them are true — I supposed that everything that had ever entered my mind was no more real than the illusions of a dream.
But then, immediately, I noticed something remarkable. Even as I was trying to think that everything was false, there was one thing that had to be true: the very fact that I was thinking this. I observed that the truth "I think, therefore I am" was so rock-solid, so absolutely certain, that no skeptic's argument, no matter how extreme, could possibly shake it. And so I concluded, without any hesitation, that I could accept this as the first principle of the philosophy I was searching for.
Next, I carefully examined what I was. I realized that I could imagine having no body. I could imagine that there was no world, no physical place where I existed. But I could not imagine that I did not exist — because the very act of doubting the reality of other things proved, clearly and certainly, that I existed. On the other hand, if I had simply stopped thinking, then even if everything I had ever imagined were actually real, I would have had no reason to believe that I existed. From this I concluded that I am a substance whose entire essence or nature consists in nothing but thinking, and which doesn't need any place to exist, nor does it depend on any material thing. In other words, the "I" — that is, the mind by which I am who I am — is completely distinct from the body. It's even easier to know than the body. And even if the body didn't exist at all, the mind would still be everything that it is.
After this, I investigated what makes a proposition true and certain in general. Since I had found one truth that I knew with certainty, I figured I should also be able to identify what gave it that certainty. When I looked at the statement "I think, therefore I am," I noticed that the only thing that assured me of its truth was this: I could see very clearly that in order to think, it is necessary to exist. So I adopted a general rule: everything we perceive very clearly and distinctly is true. The only difficulty lies in figuring out exactly which of our perceptions count as clear and distinct.
After that, I reflected on the fact that I had doubts — and that therefore my being was not entirely perfect. (It's obviously a greater perfection to know something than to doubt it.) This led me to ask: where did I get the ability to think of something more perfect than myself? I clearly recognized that this idea must have come from some nature that actually was more perfect than mine.
As for my ideas of other things outside of me — the sky, the earth, light, heat, and a thousand others — I wasn't as puzzled about where those came from. Since I didn't see anything in them that made them seem superior to me, I could believe that if they were true, they depended on my own nature (insofar as it had some degree of perfection), and if they were false, they came from nothing — that is, they existed in me only because of some imperfection in my nature.
But that explanation couldn't work for the idea of a being more perfect than myself. Getting that idea from nothing was obviously impossible. And since it's just as contradictory for something more perfect to be produced by and depend on something less perfect as it is for something to come from nothing, I couldn't have gotten this idea from myself either. The only remaining option was that it had been placed in me by a nature truly more perfect than mine — a nature that possessed within itself every perfection I could conceive of. In a word: God.
I added this further thought: since I was aware of some perfections that I didn't possess, I was clearly not the only being in existence. (I'll use the traditional philosophical terminology here, if you don't mind.) On the contrary, there had to be some other, more perfect being on whom I depended and from whom I had received everything I had. Here's why: if I had existed alone, independent of any other being — if I had gotten from myself even the small amount of perfection I actually possessed — then by the same reasoning, I should have been able to give myself all the rest of the perfection I knew I lacked. I could have made myself infinite, eternal, unchanging, all-knowing, all-powerful — in short, I could have given myself all the perfections I could recognize in God. Which is absurd.
For in order to understand God's nature (whose existence the preceding arguments have established), as far as my own nature allowed, I only needed to consider all the qualities of which I had some idea and ask whether possessing them was a mark of perfection. I was confident that none that indicated any imperfection belonged to God, and that all the rest did. So I saw that doubt, inconsistency, sadness, and things like that could not be found in God, since I myself would have been glad to be free of them. Beyond this, I had ideas of physical, material things. Even if I supposed I was dreaming and that everything I saw or imagined was false, I still couldn't deny that these ideas existed in my thoughts.
But since I had already clearly recognized that thinking nature is distinct from physical nature, and since I saw that anything composed of parts is a sign of dependency, and that dependency is clearly a kind of imperfection, I concluded that being composed of two natures could not be a perfection in God. Therefore, God was not composed of mind and body. And if there were any bodies in the world, or any minds, or any other natures that were not wholly perfect, their existence depended on God's power in such a way that they could not continue to exist without him for a single moment.
From there, I was eager to search for more truths. I turned my attention to the objects of geometry, which I conceived as a continuous body or space extending infinitely in length, width, and height (or depth), divisible into parts of different shapes and sizes that could be moved and rearranged in all sorts of ways — since this is what geometers take as their subject matter. I reviewed some of their simplest proofs.
First, I noticed that the great certainty everyone agrees these proofs have is based solely on the fact that they are clearly conceived, following the rules I had already established. Second, I noticed that there was nothing in these proofs that guaranteed the actual existence of their objects. For example, when I considered a triangle, I could clearly see that its three angles necessarily add up to two right angles. But that didn't prove that any triangle actually exists in the world. On the other hand, when I turned back to my idea of a Perfect Being, I found that existence was contained in that idea in the very same way that the property of having angles adding up to two right angles is contained in the idea of a triangle — or the way all points on the surface of a sphere are equidistant from its center. In fact, it was even clearer than those examples. Therefore, it is at least as certain that God — this Perfect Being — exists as any proof in geometry.
Now, the reason many people convince themselves that it's hard to know this truth — and even hard to know what their own mind really is — is that they never lift their thoughts above physical objects. They're so used to thinking only through imagination (which is a way of thinking limited to material things) that anything they can't picture seems unintelligible to them. This is clear enough from the fact that the traditional philosophers adopted as a principle that nothing is in the understanding that wasn't first in the senses — a principle that clearly doesn't apply to the ideas of God and the soul, which have never been in the senses. It seems to me that people who try to use their imagination to grasp these ideas are doing essentially the same thing as if they tried to use their eyes to hear sounds or smell odors. There is actually this difference: the sense of sight is no less reliable than smell or hearing. But neither imagination nor the senses can give us certainty about anything unless our understanding — our reason — steps in to verify it.
Finally, if there are still people who aren't convinced of God's existence and the soul's reality by the arguments I've given, I want them to know this: all the other things they think they're more certain of — that we have a body, that stars and an earth exist, and so on — are actually less certain. Yes, we have what you might call a practical certainty about these things, strong enough that it seems crazy to doubt them. But at the same time, no reasonable person can deny that, when it comes to absolute metaphysical certainty, there is room for doubt. After all, we can imagine, while asleep, that we have a different body, that we see different stars and a different earth — when none of those things exist. How do we know that the thoughts we have in dreams are any less true than the ones we have while awake, when our dream-thoughts are sometimes just as vivid and detailed?
Let the most brilliant minds study this question for as long as they like. I don't believe they can give a reason sufficient to remove this doubt unless they assume that God exists. For consider: even the principle I've already adopted as a rule — that everything we perceive clearly and distinctly is true — is only certain because God exists, because God is a Perfect Being, and because everything we have comes from him. It follows that our ideas and concepts, to the extent that they are clear and distinct, are real and come from God, and must therefore be true. On the other hand, when we have ideas that contain some falsehood, this can only be the case with ideas that are somewhat confused and obscure — ideas that, to that extent, participate in nothingness. They are confused in us because we are not entirely perfect. And it's clear that it would be just as contradictory for falsehood or imperfection (as imperfection) to come from God as it would be for truth or perfection to come from nothing. But if we didn't know that everything real and true in us comes from a Perfect and Infinite Being, then no matter how clear and distinct our ideas might be, we would have no grounds for being sure they were true.
But once our knowledge of God and the soul has given us confidence in this rule, we can easily see that our waking thoughts should not be called into question just because of what happens in our dreams. If someone had a truly distinct idea while asleep — say a geometer discovered a new proof in a dream — the fact that they were asleep wouldn't make the proof any less valid. As for the most common error in dreams, which is that they present objects to us in the same way our senses do when we're awake — this isn't really a problem, because it rightly makes us suspicious of what the senses tell us. After all, we're frequently deceived in the same way while awake: people with jaundice see everything as yellow, and distant stars and objects appear much smaller than they really are.
In the end, whether awake or asleep, we should never let ourselves be convinced of anything except by the evidence of our reason. Notice that I say our reason — not our imagination or our senses. For example, although we can clearly see the sun, we shouldn't conclude that it's only as big as it looks. And we can vividly imagine a lion's head stuck on a goat's body, but that doesn't mean a chimera actually exists. Reason doesn't tell us that what we see or imagine is necessarily real. But it does tell us that all our ideas must contain some truth — because otherwise, God, who is perfectly truthful, would not have placed them in us. And because our reasoning is never as clear or as thorough during sleep as when we're awake (even though our imagination can sometimes be just as vivid in dreams), reason tells us this: since not all our thoughts can be true — because we aren't perfect — the ones that are true will inevitably be found among our waking experiences rather than our dreams.
I would have been happy to lay out the whole chain of truths I worked out from these first principles. But doing that properly would have meant diving into many questions that scholars love to argue about, and I have no desire to get caught up in those battles. So I think it's better to hold back the full account and just describe in general terms what those truths are. That way, the more thoughtful readers can judge for themselves whether a detailed treatment would be worth the effort.
I have always held firm to my original resolution: to start from no principle other than the one I just used to prove the existence of God and the soul, and to accept nothing as true unless it seemed clearer and more certain to me than the proofs of geometry ever had. And yet I can say that not only did I find ways to answer, in a short time, all the major questions that philosophy usually deals with, but I also discovered certain laws that God has built into nature. He has planted the ideas of these laws so deeply in our minds that, once we reflect on them carefully, we cannot doubt that they hold true for everything that exists or happens in the world. What's more, by tracing how these laws connect to one another, I believe I've uncovered many truths more useful and more important than anything I had ever learned before — or even expected to learn.
Now, I tried to explain the most important of these discoveries in a treatise that certain considerations have prevented me from publishing. So the best way to share what I found is to give a summary of that treatise here. My plan was to include in it everything I thought I knew about the nature of physical objects before I sat down to write. But just as painters, unable to show every side of a solid object on a flat surface, pick one main face to light up and throw the rest into shadow — letting those other faces show only as glimpsed from the angle of the main one — so I, afraid I couldn't fit everything in my mind into a single work, decided to focus mainly on light. From there I would add something about the sun and the fixed stars, since light mostly comes from them; the heavens, since they carry it; the planets, comets, and earth, since they reflect it; and especially all the objects on the earth, since they are either colored, transparent, or luminous. Finally I would discuss human beings, since we are the ones who observe all these things.
To give myself more freedom in this discussion — without having to accept or argue against the established opinions of the scholars — I decided to set aside the real world entirely. Instead I would talk about what would happen in a brand new world, if God were to create, somewhere in infinite space, enough matter to make one, and if He stirred up the different parts of that matter every which way so that the result was a chaos as wild as any poet ever imagined. Then suppose He did nothing more than give nature His usual support and let it operate according to the laws He had established. Under this assumption, I first described this matter, and I tried to present it so clearly that nothing could be more plain and understandable, except what I'd already said about God and the soul. I specifically assumed that this matter had none of those "forms" or "qualities" that the universities argue about so endlessly — nothing, in fact, that isn't so naturally obvious to our minds that no one could even pretend not to know it. Beyond that, I laid out the laws of nature. Using no principle other than God's infinite perfection, I worked to demonstrate every law that could be in any way doubtful, and to show that these laws are so fundamental that even if God had created many worlds, there could not be a single one where these laws did not hold.
After that, I showed how the greater part of this chaotic matter would, in accordance with these laws, sort itself out and arrange itself into something resembling our heavens. Meanwhile, some parts of it would form an earth, some would become planets and comets, and others would become a sun and fixed stars. Here I made a digression on the subject of light, explaining at length what the nature of light must be as found in the sun and stars, how it travels across the vast distances of the heavens in an instant, and how it bounces off the planets and comets back toward the earth. I also added a good deal about the makeup, position, movements, and qualities of these heavens and stars — enough to show that nothing we can observe in our actual heavens and stars is any different, or at least need be any different, from what you'd see in the system I was describing.
From there I moved on to the earth itself. I showed how, even though I had deliberately assumed that God gave no weight to the matter it was made of, every part of that matter would still tend toward its center. I showed how, with water and air on its surface, the arrangement of the heavens and celestial bodies — especially the moon — would cause tides very much like the ones we see in our actual oceans, and also a general current of both water and air flowing from east to west, just like the one we observe between the tropics. I explained how mountains, seas, springs, and rivers could form naturally, how metals could be produced in mines, and how plants could grow in the fields. In general, I showed how all the substances we call "mixed" or "composite" could be generated.
Among these topics, since I knew of nothing besides the stars that produces light except fire, I spared no effort in explaining everything about fire's nature — how it is produced and sustained, how it sometimes creates heat without light and light without heat, how it can give different colors to different objects and produce all sorts of other effects, how it melts some things and hardens others, how it can consume nearly everything or reduce it to ashes and smoke. And finally, how from those very ashes, through sheer intensity, fire creates glass. This transformation of ashes into glass struck me as one of the most remarkable things in all of nature, and I took special pleasure in describing it.
I was not, however, about to conclude from any of this that our actual world was created in the way I described. It's far more likely that God made it, right from the start, just as it was meant to be. But here is something that both I and most theologians agree on: the act by which God sustains the world right now is the same act by which He originally created it. So even if He had started with nothing but chaos, as long as He established the laws of nature and gave nature His support to operate as it normally does, we can believe — without any disrespect to the miracle of creation — that purely physical things could, over time, have become exactly what we see today. And honestly, it's much easier to understand the nature of things when you watch them come into being step by step than when you consider them only in their final, finished state.
From my description of lifeless objects and plants, I moved on to animals, and especially to human beings. But since I didn't yet have enough knowledge to discuss them the way I had discussed everything else — that is, by tracing effects back to their causes and showing what raw materials nature uses and how — I took a different approach. I imagined that God had formed a human body exactly like one of ours, identical in both its outward shape and its internal structure, made from the same kind of matter I had been describing. But at first, I placed no rational soul in it and no other principle to serve as a "vegetative" or "sensitive" soul. The only thing I added was a fire without light kindled in the heart — the kind of fire I had already described, no different from the heat that builds up in a pile of damp hay or the warmth that drives fermentation in freshly pressed grape juice.
When I examined what functions this body could perform on the basis of this setup alone, I found exactly those functions that exist in us without any involvement of thought — everything that doesn't depend on the soul, meaning that part of us whose essential nature, as I said before, is thinking. These are the very functions in which animals resemble us completely. But among them I could not find any of those functions that depend on thought alone and belong exclusively to us as human beings. I did discover all of those, however, as soon as I supposed that God had created a rational soul and joined it to this body in the particular way I went on to describe.
But to show how I handled this topic, I want to explain here the motion of the heart and arteries. This is the first and most basic motion we observe in animals, and understanding it makes it easy to figure out what to think about all the rest. To help readers who aren't familiar with anatomy follow what I'm about to say, I'd suggest that before reading further, they go watch someone dissect the heart of a large animal that has lungs (since any such heart is similar enough to a human one). Have someone show you its two chambers, or ventricles.
First, the chamber on the right side. Two large tubes connect to it. One is the hollow vein (the vena cava), which is the main reservoir of the blood and the trunk, so to speak, of a tree whose branches are all the other veins in the body. The other is the arterial vein (which is really an artery, despite its misleading name). This vessel originates in the heart and, after leaving it, splits into many branches that spread throughout the lungs.
Second, the chamber on the left side. It also has two tubes of equal or even greater size. One is the venous artery (also misleadingly named, since it's really a vein). It comes from the lungs, where it divides into many branches interwoven with those of the arterial vein and with the windpipe, which is the tube through which we breathe. The other is the great artery (the aorta), which exits the heart and sends its branches all over the body.
I would also want these observers to be carefully shown the eleven small flaps that act as valves, opening and closing the four openings of these two chambers. Three are at the entrance from the hollow vein: they are positioned so that they freely allow blood to flow into the right ventricle but completely prevent it from flowing back out. Three more are at the entrance to the arterial vein, arranged the opposite way: they let blood pass easily from the right ventricle into the lungs but block it from returning. Similarly, two are at the opening of the venous artery, allowing blood to flow from the lungs into the left ventricle but not back again. And three are at the mouth of the great artery, letting blood flow out from the heart but preventing any backflow. There's no mystery about why some openings have two valves and others have three: the opening of the venous artery is oval-shaped and can be sealed shut with just two flaps, while the round openings need three.
Also notice that the great artery and the arterial vein have much harder, thicker walls than the venous artery and the hollow vein. And notice that the hollow vein and the venous artery widen out before they enter the heart, forming two pouches called the auricles of the heart, made of the same kind of tissue as the heart itself. One more thing: there is always more heat in the heart than in any other part of the body. This heat is strong enough to make any drop of blood that flows into the heart's chambers quickly expand and puff up, just the way any liquid does when you let it fall drop by drop into a very hot vessel.
Once you understand all of this, explaining how the heart moves becomes straightforward. When the heart's chambers are not full of blood, blood inevitably flows in — from the hollow vein into the right chamber and from the venous artery into the left — because these two vessels are always full of blood and their openings into the heart can't stay closed at that point. But as soon as two drops of blood have entered, one into each chamber (and these drops are fairly large, since the openings are wide and the vessels are full), those drops are immediately heated and expanded by the warmth they encounter. This causes the whole heart to swell. At the same time, the expanding blood pushes shut the five small valves at the entrances of the two vessels it came from, cutting off any further inflow. Then, growing still more expanded, the blood forces open the six small valves at the exits of the other two vessels, causing all the branches of the arterial vein and the great artery to inflate almost simultaneously with the heart.
Right after that, the heart begins to contract, and so do the arteries, because the blood that entered them has cooled down. The six exit valves close, and the five entrance valves open again, admitting two fresh drops of blood, which cause the heart and arteries to expand all over again, just as before. And because the blood entering the heart passes through those two pouches called the auricles, their motion is the opposite of the heart's: when the heart expands, the auricles contract.
Now, in case anyone who isn't used to mathematical proofs and doesn't know the difference between a real demonstration and a plausible-sounding story is tempted to dismiss all of this without examining it, let me point out: the motion I've just described follows just as necessarily from the arrangement of the parts you can see in the heart with your own eyes, from the heat you can feel with your fingers, and from the nature of the blood as we know it from experience, as the motion of a clock follows from the power, position, and shape of its counterweights and gears.
But if someone asks how the blood in the veins doesn't run out, since it's constantly flowing into the heart, and why the arteries don't overflow, since all the blood that passes through the heart goes into them — well, I need only mention what was discovered by an English physician who deserves the honor of having broken new ground on this topic. He was the first to explain that there are many tiny passages at the far ends of the arteries, through which the blood the heart sends out passes into the small branches of the veins and then returns to the heart. The blood's path, in other words, is a continuous loop — a perpetual circulation.
There is plenty of evidence for this. Surgeons see it every day. When they tie a band around the arm, moderately tight, above the spot where they open a vein, the blood flows out more freely than it would without the band. But the opposite would happen if they tied it below — that is, between the hand and the cut — or if they cinched the band very tightly above the cut. Here's why: a moderately tight band is enough to prevent the blood already in the arm from flowing back to the heart through the veins, but it can't stop fresh blood from arriving through the arteries, because the arteries lie deeper than the veins and have tougher walls that are harder to compress. Also, blood coming from the heart through the arteries pushes toward the hand with greater force than it pushes back toward the heart through the veins. Since the blood in the arm escapes through the opening in the vein, there must be small passages below the band — near the tips of the fingers — through which blood can get from the arteries into the veins.
This physician also convincingly demonstrated the circulation of the blood by pointing to the existence of small flaps, like tiny valves, positioned at various points along the veins. These valves allow blood to flow from the extremities back to the heart but do not allow it to flow the other direction, from the center of the body outward. He further showed that if you cut a single artery, even a small one, nearly all the blood in the body can drain out of it in a very short time — even if the artery has been tied off right next to the heart and cut between the heart and the ligature, so there's no possibility the blood could be coming from anywhere but the heart.
There are many other facts that confirm that the true cause of the blood's motion is the one I've described. First, consider the difference between blood from the veins and blood from the arteries. This difference can only be explained by the fact that blood, after passing through the heart, has been heated and thinned out — almost distilled, you might say. It's lighter, more lively, and warmer right after leaving the heart (in the arteries) than it was just before entering it (in the veins). And if you pay close attention, you'll notice that this difference is most obvious near the heart and becomes less pronounced the farther from it you get.
Second, the walls of the arterial vein and the great artery are much tougher and thicker than those of the other vessels, which makes sense because the blood pushes against them with more force.
Third, why would the left ventricle and the great artery be wider and larger than the right ventricle and the arterial vein, if not for this reason: the blood that arrives through the venous artery has already passed through the lungs after leaving the heart, so it's thinner and expands more readily and to a greater degree than the blood coming straight from the hollow vein?
Fourth, what can physicians learn from taking someone's pulse, if not that the blood's nature determines how much and how quickly the heart's heat can cause it to expand?
Fifth, if you ask how the heart's heat gets distributed to the rest of the body, the answer has to be: through the blood. The blood passes through the heart, gets reheated there, and then spreads to every part of the body. That's why, when blood is cut off from any limb, the heat leaves with it. Even if the heart were as hot as a piece of glowing iron, it couldn't warm your hands and feet unless it were constantly sending fresh heated blood to them.
From this we can also see the true purpose of breathing. Its job is to bring enough fresh air into the lungs to cause the blood flowing in from the right ventricle — blood that has been heated and turned almost into a vapor — to thicken again and turn back into proper blood before it enters the left ventricle. Without this process, the blood wouldn't be fit to fuel the fire that burns in the heart. This is confirmed by the observation that animals without lungs have only one chamber in their hearts, and that in unborn babies, who can't yet use their lungs, there's a hole that lets blood flow straight from the hollow vein into the left ventricle, plus a tube that carries it from the arterial vein directly into the great artery, bypassing the lungs entirely.
Next, how could digestion happen in the stomach without the heart sending it heat through the arteries, along with some of the more fluid parts of the blood to help dissolve the food? And isn't it easy to understand how the juice of food gets converted into blood when you consider that it passes through the heart — being heated and distilled there — perhaps more than a hundred or two hundred times a day?
As for nutrition and the production of the body's various fluids, what more do we need to explain than this: the force with which the blood, expanding, pushes from the heart toward the tips of the arteries causes some of its particles to stop and settle in the parts of the body they reach, taking the place of others that get pushed out. Depending on the shape, size, and arrangement of the tiny pores they encounter, some blood particles end up in some parts of the body and others end up elsewhere — just as different-sized holes in a sieve sort different kinds of grain.
Finally, and most remarkable of all, there are the animal spirits. These are like an extremely fine wind, or more accurately, like a very pure and lively flame. They rise in large quantities from the heart to the brain, then flow through the nerves into the muscles, giving movement to every part of the body. The most active and penetrating parts of the blood are best suited to make up these spirits, and they naturally head toward the brain for a simple reason: the arteries that carry them there run in the most direct line from the heart. And according to the laws of mechanics — which are the same as the laws of nature — when many particles are heading for the same destination and there isn't room for all of them (as is the case with blood flowing from the left ventricle toward the brain), the weaker and slower ones get pushed aside by the stronger ones, so that only the strongest and most energetic ones arrive.
I had explained all of these matters in considerable detail in the treatise I once planned to publish. After the topics I've described, I had gone on to show what the structure of the nerves and muscles must be in order for the animal spirits to move the body's limbs — the way we see severed heads still moving and biting the ground even though they are no longer alive. I explained what changes must happen in the brain to produce waking, sleep, and dreams. I described how light, sounds, smells, tastes, heat, and all the other qualities of external objects imprint different ideas on the brain through the senses, and how hunger, thirst, and other internal drives do the same. I explained what should be understood by the "common sense" — the faculty where all these sense impressions are received — and by memory, which stores them, and by imagination, which can rearrange them in various ways and combine them into new ideas. I showed how, through these same mechanisms, the animal spirits distributed through the muscles can cause the body's limbs to move in as many different ways, and as appropriately — whether in response to objects that stimulate the senses or to internal drives — as our own bodies can move without any direction from the will.
None of this should seem strange to anyone who knows how many different movements the various automata, or mechanical devices, built by human engineers can perform, using only a handful of parts compared to the enormous number of bones, muscles, nerves, arteries, veins, and other components found in an animal's body. Think of the animal body as a machine made by the hands of God — a machine incomparably better designed, and capable of far more amazing movements, than anything human inventors have ever built.
And here I paused to make a particularly important point. If there were machines that perfectly resembled an ape or some other animal in their organs and outward appearance, we would have absolutely no way of telling that they were any different from the real animals. But if there were machines that looked like human bodies and could imitate our actions as closely as is practically possible, we would still have two sure-fire tests to tell that they were not real human beings.
The first test is language. These machines could never use words or other signs and arrange them meaningfully the way we do to communicate our thoughts to others. You could certainly build a machine that says certain words — even one that responds to specific physical triggers, like crying "Ouch!" when touched in a certain spot or asking "What do you want?" when touched in another. But you could not build one that arranges words on its own to give a sensible reply to whatever is said to it. Even the least intelligent among us can do that, but no machine could.
The second test is versatility. Even though such machines might do certain things as well as we do, or even better, they would inevitably fail at others. And from those failures you could see that they were not acting from understanding but only from the arrangement of their parts. Reason is a universal tool, useful in every kind of situation. But a machine's parts need to be specifically arranged for each specific task. It would be practically impossible to build a machine with enough different mechanisms to handle every situation in life the way our reason enables us to.
These same two tests also let us tell the difference between humans and animals. It's remarkable that no human being — no matter how dull or slow, not even those with severe mental disabilities — is unable to string words together and construct some statement that communicates their thoughts. And on the other hand, no animal, no matter how talented or well-trained, can do the same. This isn't because animals lack the right physical equipment. We know that magpies and parrots can produce words just like us, and yet they can never speak in a way that shows they understand what they're saying. Meanwhile, people who are born deaf and mute, and who therefore lack even more of the usual organs of speech than animals do, routinely invent their own signs to express their thoughts to the people around them.
This proves not just that animals have less reason than humans, but that they have no reason at all. After all, very little reason is needed to be able to speak. And since we see variation in ability among animals of the same species, just as among humans, and since some animals learn better than others, it's inconceivable that the smartest parrot or ape of its kind couldn't match the speech of the most limited human child — unless the nature of the animal's soul is entirely different from ours.
We should not confuse real speech with the natural cries and movements that express emotions, which machines can imitate just as easily as animals can produce. And we shouldn't fall for the old idea that animals really do talk but we just can't understand them. If that were true, then since they have many of the same organs we do, they could communicate with us as easily as they communicate with each other.
It's also worth noting that while many animals outperform us in certain specific tasks, those same animals show zero ability in many other areas. The fact that they sometimes do better than us doesn't prove they have minds — if it did, we'd have to say they were smarter than all of us and could beat us at everything. Instead, it proves they don't have reason at all. It's their nature acting through the arrangement of their organs, not their intelligence. A clock made of nothing but wheels and weights can count the hours and measure time more precisely than we can, with all our intelligence.
After all of this, I described the rational soul and showed that it cannot possibly arise from physical matter the way everything else I had discussed can. It must be specifically created. And it is not enough for it to be housed in the human body the way a pilot sits in a ship, useful only for steering the limbs. No — it has to be joined and united with the body far more intimately than that, in order to have sensations and desires like ours and so to make up a real, complete human being.
I went into the subject of the soul at great length, because it is of the utmost importance. After the error of denying God's existence — which I believe I have already sufficiently answered — there is no mistake more dangerous to people's moral well-being than the belief that animal souls are the same as ours. If that were true, we would have nothing to hope for or fear after this life, no more than flies or ants do. But once we understand how profoundly different our souls are from those of animals, we can much better appreciate the reasons for believing that the soul is completely independent of the body and therefore not destined to die with it. And since we see nothing else that could destroy it, we are naturally led to conclude that the soul is immortal.
Three years had passed since I finished the treatise containing all these topics, and I was starting to revise it for publication when I learned some troubling news. Certain people whose authority I deeply respect — people whose influence over my actions is almost as strong as my own reason's influence over my thoughts — had condemned a particular doctrine in physics that another individual had recently published. I won't say I agreed with that doctrine, but I will say that before the censure, I had noticed nothing in it that seemed harmful to religion or the state, and nothing that would have stopped me from writing it myself if reason had convinced me it was true. This made me afraid that somewhere in my own work, I too might have strayed from the truth — despite always being extremely careful not to believe any new idea unless I had rock-solid proof, and not to write anything that could harm anyone. This was enough to make me change my plans about publishing. Although I'd had strong reasons for wanting to publish, my natural dislike of writing books made it easy to find other reasons not to. And these reasons, on both sides, are worth laying out here — partly for my own sake, and partly because the public might want to know them.
I had never thought much of what came from my own mind. As long as the only benefit I got from my Method was solving some theoretical puzzles, or guiding my behavior according to the principles it taught me, I never felt obligated to publish anything about it. When it comes to how people should live, everyone already thinks they're an expert — there'd be as many reformers as there are people, if anyone besides God-appointed rulers or genuine prophets were allowed to take on that job. And although my own speculations pleased me greatly, I figured other people had their own ideas that pleased them just as much.
But as soon as I'd arrived at some general principles about physics and started testing them against specific problems, I saw how far they could take us and how different they were from every principle used up to that point. At that moment, I realized I couldn't keep them to myself without seriously sinning against the duty we all have to promote the common good of humanity as much as we can.
Here's why. Through these principles, I saw that it was possible to gain knowledge enormously useful for everyday life. Instead of the purely theoretical philosophy taught in the universities, we could develop a practical philosophy. By understanding the forces and behaviors of fire, water, air, the stars, the heavens, and everything else around us — understanding them as clearly as we understand the trades of our craftspeople — we could put that knowledge to work for us in the same way, and make ourselves, in effect, the masters and owners of nature.
And this isn't just about inventing countless technologies that would let us enjoy the fruits of the earth without so much labor. It's especially about preserving health, which is, without a doubt, the first and most fundamental blessing in life. The mind depends so deeply on the condition of the body's organs that if there's any way to make people wiser and more capable than they've been so far, I believe it has to come through medicine. Now, it's true that medicine as it currently stands doesn't contain much of real use. But without meaning to insult it, I'm confident that no one — not even practicing doctors — would deny that everything we currently know about medicine is almost nothing compared to what's left to discover. We could free ourselves from countless diseases of both body and mind, and perhaps even from the decline of old age, if we had thorough enough knowledge of their causes and of all the remedies nature has provided.
Since I had planned to spend my whole life searching for this crucially important science, and since I had found a path that seemed to guarantee success for anyone who followed it — unless they were blocked by a short life or a lack of experiments — I decided the best strategy against both of those obstacles was this: share everything I'd found with the public, and encourage people of greater talent to push further, each contributing experiments according to their abilities and interests, and sharing all their discoveries. That way, the last person in the chain could pick up right where the ones before them left off, and by linking together the lives and work of many people, we could collectively go much further than any one person could alone.
I also noticed something important about experiments: they become more and more necessary the further you advance in knowledge. At the beginning, it's better to work with what our senses naturally show us — things we can't help but notice as long as we reflect on them at all — rather than chasing rare and obscure phenomena. The reason is that unusual phenomena tend to mislead us as long as we don't yet understand the more ordinary ones, and the conditions that produce them are almost always so specialized and tiny that they're extremely hard to detect.
Here's the approach I followed. First, I tried to identify the most general principles — the first causes of everything that exists or could exist in the world — taking nothing as my starting point except God himself, who created it all, and deriving these principles from nothing other than certain seeds of truth that naturally exist in our minds. Second, I examined the first and most basic effects that could be deduced from these causes. Through this process, I found I could account for the heavens, the stars, the earth, and even things on the earth like water, air, fire, and minerals — the most common and simplest things, and therefore the easiest to understand.
But when I tried to move on to more specific things, so many different objects presented themselves that I realized the human mind couldn't possibly distinguish all the forms and types of things on earth from the infinite number of other forms God could have placed there. The only way to connect them to our use was to work backward from effects to causes and to carry out many particular experiments.
When I reviewed everything my senses had ever presented to me, I can honestly say I never encountered anything I couldn't satisfactorily explain using the principles I'd discovered. But I also have to admit that nature's power is so vast, and these principles are so simple and general, that almost any individual effect can be deduced from them in many different ways. My biggest difficulty is usually figuring out which of those ways actually explains the effect. The only way past this difficulty is to find experiments whose results would differ depending on which explanation is correct.
As for what remains, I can now see clearly enough what experiments need to be done. But I also see that they're so numerous that neither my own hands nor my income — even if it were a thousand times what it is — would be enough for all of them. So the more experiments I'm able to perform, the more progress I'll make in understanding nature. This is exactly what I'd hoped to communicate in that treatise: to show so clearly the advantage the public would gain that I'd inspire everyone who genuinely cares about the common good — people who are truly virtuous, not just pretending to be — both to share whatever experiments they'd already done and to help me with the ones still needed.
But since then, other reasons have led me to change my mind. I came to think that I should indeed write down all the results I considered important, as soon as I'd verified their truth, and take just as much care with them as if I were planning to publish. This approach had several advantages. It gave me more incentive to examine everything thoroughly — because we always scrutinize things more carefully when we expect others to read them than when we write only for ourselves. (Often enough, what seemed true when I first thought of it turned out to be false when I tried to put it in writing.) It also meant I wouldn't miss any opportunity to serve the public interest. And if my writings turned out to have any value, people who found them after my death could use them however they saw fit.
But I was firmly resolved not to let them be published during my lifetime. I didn't want the controversies and objections they'd provoke — or even whatever reputation they might bring me — to cost me the time I'd set aside for my own education. It's true that everyone should promote the good of others as far as they can, and that being useful to no one means being truly worthless. But it's also true that our concerns should extend beyond the present moment. Sometimes it's better to skip doing something that might benefit people alive now if you have your sights set on accomplishments that will benefit future generations far more.
And honestly, I'm happy for people to know that the little I've learned so far is almost nothing compared to everything I don't yet know — and that I haven't given up hope of learning it. People who gradually discover truths in the sciences are a lot like people who are becoming rich: the richer they get, the easier it is to make large gains, while back when they were poor, even small gains were a struggle. Or think of army commanders, whose forces usually grow after each victory, and who need more skill to hold their remaining troops together after a defeat than they need to capture towns and provinces after a win. Because anyone who tries to overcome all the difficulties and errors that stand between them and the truth is genuinely fighting a battle. And anyone who accepts a false belief on a matter of any importance has lost that battle — and it takes much more skill to recover your position afterward than to make great advances once you're standing on solid, well-established principles.
As for myself, if I've managed to discover any truths in the sciences (and I hope that what's in this volume will show I've found some), I can say they're just the results of five or six major difficulties I've overcome — encounters I counted as battles where victory went my way. I'll even go so far as to say I believe all I need to fully achieve my goals is to win two or three more such victories, and that I'm not so old that I can't reasonably expect enough time for this. But the greater my hope of using my remaining time well, the more I feel bound to protect it carefully. And I'd have a lot to steal it from me if I published the principles of my physics. Although they're all nearly self-evident — you basically just need to understand them to agree with them — and although I expect to be able to demonstrate every one of them, they can't possibly align with everyone else's diverse opinions. I could see that I'd constantly be pulled away from my main project by the arguments they'd inevitably provoke.
Now, someone might say these objections would actually be useful — they'd alert me to my mistakes, and if my ideas have any merit, they'd help others understand them better. Plus, since many people can see more than one, others who start using my principles might help me with their own discoveries. I appreciate the thought, but here's the reality. Even though I know I'm extremely prone to error and almost never trust my first ideas, my experience with objections hasn't given me much reason to expect profit from them. I've already had plenty of feedback — from people I considered friends, from people who were presumably indifferent to me, and even from people whose jealousy and hostility I knew would drive them to find whatever fault my friends' bias might have hidden. And yet it has rarely happened that anyone raised an objection I hadn't already considered myself, unless it was something completely beside the point. I've never encountered a single critic of my views who didn't seem to me either less careful or less fair-minded than I was.
Furthermore, I've never seen any unknown truth come to light through the kind of debates practiced in the universities. Everyone fights for victory. Everyone works harder at making their position look plausible than at genuinely weighing both sides of the question. And people who've spent years being good debaters don't become better judges because of it.
As for the benefit others might get from hearing my ideas: it couldn't be very great right now, because I haven't yet taken them far enough to be applied in practice. And I think I can say without boasting that if anyone is going to take them that far, it would have to be me rather than someone else. Not that there aren't plenty of minds in the world that are incomparably superior to mine — but you can never grasp something and truly make it your own as well when you've learned it from someone else as when you've discovered it yourself.
This is so true of the present subject that even though I've often explained some of my ideas to very sharp people who seemed to understand them perfectly while I was talking, when they repeated those ideas back, they'd almost always changed them so much that I could no longer recognize them as mine. And while I'm on this topic, let me take this opportunity to ask future readers: please never believe that something came from me unless I published it myself. I'm not at all surprised by the wild ideas attributed to the ancient philosophers whose own writings we no longer have. I don't think those philosophers were actually absurd — they were among the most brilliant people of their time — but rather that their ideas have been badly misrepresented over the centuries.
You can see, in fact, that hardly any of their followers ever surpassed them. And I'm quite sure that the most devoted present-day followers of Aristotle would consider themselves lucky to have as much knowledge of nature as he had, even if it meant they could never go beyond it. They're like ivy that never tries to climb above the tree supporting it, and often actually grows back downward once it reaches the top. Similarly, these people actually become less wise than they'd be if they stopped studying altogether — because they're not satisfied with knowing everything their master intelligibly explained, but want to find in him the answers to countless questions he never said a word about and probably never even considered.
Their way of doing philosophy, though, is well suited to people of below-average ability. The obscurity of the distinctions and principles they use lets them talk about anything with total confidence, as if they really knew what they were saying, and defend their positions against even the most clever opponents without anyone being able to prove them wrong. In this respect, they're like a blind person who, in order to fight on equal terms with someone who can see, insists on dragging the fight to the bottom of a pitch-dark cave. And I'd say these people actually have an interest in my not publishing my principles of philosophy — because my principles are so simple and self-evident that publishing them would be like throwing open the windows and flooding that dark cave with daylight.
But even truly talented people have no great reason to be anxious about learning these principles. If what they want is to be able to talk about everything and build a reputation for learning, they'll achieve that more easily by settling for the appearance of truth — which can be found without much difficulty on any topic — than by seeking truth itself, which reveals itself only slowly, only in certain areas, and forces us to honestly admit our ignorance when we speak of everything else.
If, however, they prefer actually knowing some real truths over the vanity of appearing to know everything — and such knowledge is undoubtedly far preferable — and if they choose to follow a path like mine, they don't need me to say anything more than what I've already said in this Discourse. If they're capable of going further than I have, they'll certainly be able to discover for themselves everything I think I've found. Since I've always investigated things in order, whatever's left to discover is naturally more difficult than what I've already managed to find, and there'd be much less satisfaction in learning it from me than in discovering it themselves.
Besides, the habit they'll develop — by starting with what's easy and then moving step by step to the harder things — will do them more good than all my instruction possibly could. I'm convinced that if someone had taught me from childhood all the truths I've since worked out on my own, and I'd learned them without any effort, I'd probably never have known anything beyond them. I certainly wouldn't have developed the ability I think I now have: consistently discovering new truths in proportion to how much effort I put into the search. In short, if there's any work in the world that absolutely cannot be finished as well by someone else as by the person who started it, it's the work I'm doing.
It's true that when it comes to experiments, one person can't possibly do them all. But the only help he can really use — besides his own hands — is that of skilled workers or technicians he can pay, people whose hope of being paid (a highly effective motivator) will push them to do exactly as they're told. As for volunteers who offer their services out of curiosity or a love of learning, their promises generally exceed their follow-through. They sketch out grand plans, none of which ever get completed, and they'll inevitably expect to be repaid for their trouble with explanations of various problems, or at the very least with compliments and polite conversation — all of which wastes the researcher's time.
And as for experiments others have already done: even if people were willing to share them (which those who consider them trade secrets will never do), most experiments come buried in so many unnecessary details and irrelevant conditions that it's incredibly hard to separate the actual truth from everything around it. Worse, you'll find that almost all of them are either poorly described or outright false — because the people who did them wanted to see only results that confirmed their existing theories. So even if a few of those experiments happened to be useful, it still wouldn't be worth the time needed to sift through and select them.
So imagine there existed someone we were absolutely certain could make the most important and useful discoveries possible, and that everyone was eager to help him succeed. I don't see what anyone could do for him beyond covering the cost of his experiments and, for the rest, making sure no one interrupts his work. But I'm neither so full of myself as to promise anything extraordinary, nor so lost in fantasy as to imagine the public should care much about what I'm doing. On the other hand, I don't have such a small soul that I'd accept a favor people thought I didn't deserve.
All these considerations together are why, for the past three years, I was unwilling to publish that treatise, and why I even decided not to publish anything else during my lifetime that was so general or that might reveal the principles of my physics. But since then, two additional reasons have motivated me to include some specific examples here, and to give the public some account of what I've been doing and what I plan to do.
The first reason is this: if I said nothing, the many people who knew I'd originally intended to publish might imagine that my reasons for holding back were less honorable than they actually are. I'm not excessively hungry for glory — in fact, if I may say so, I'm averse to it, since I consider it a threat to the peace and quiet I value above all else. Yet at the same time, I've never tried to hide my work as though it were something criminal, and I've never gone out of my way to remain unknown. Partly because secrecy would have felt like doing myself an injustice, and partly because it would have caused me a kind of anxiety that itself would have disturbed the perfect mental tranquility I seek. And since, despite being genuinely indifferent to both fame and obscurity, I haven't been able to prevent myself from gaining a certain reputation, I've decided I owe it to myself to at least make sure I'm not spoken of badly.
The second reason is that I'm becoming more aware every day of how much my plans for self-education are being delayed by the enormous number of experiments I need but can't do by myself. Without flattering myself into thinking the public will take much interest in my work, I also don't want to fall so short of the duty I owe to myself that people in the future could rightly blame me. They might say that I could have left them many things in a much better state than I did, if only I hadn't neglected to let them know how they could have helped me accomplish my goals.
I thought it would be easy to choose some topics that wouldn't stir up too much controversy and wouldn't force me to reveal more of my principles than I wanted, but that would still clearly demonstrate what I can and can't accomplish in the sciences. Whether I've succeeded in this, it's not for me to say. I don't want to bias anyone's judgment by reviewing my own work. But I'd be pleased if people examined it, and to encourage this, I invite anyone who has objections to send them to my publisher, who'll pass them on to me so I can include my responses alongside them. That way, readers can see both the objection and my reply at once, and more easily judge where the truth lies. I don't promise to give lengthy replies in any case — only to honestly acknowledge my mistakes if I'm convinced of them, or if I don't see the error, to simply state what I think is needed to defend what I've written, without adding any new topics and getting dragged into an endless back-and-forth.
If some of the ideas I discuss at the beginning of the Dioptrics and the Meteorics seem off-putting at first because I call them "hypotheses" and don't seem to bother proving them, I ask for patient and careful reading of the whole work. I hope that will satisfy any doubts. The arguments in these treatises are so interconnected that the later conclusions are demonstrated by the earlier principles (which are their causes), and the earlier principles are in turn demonstrated by the later conclusions (which are their effects). Don't think I'm committing the logical error of circular reasoning here. Since experience confirms most of these effects beyond any doubt, the causes I deduce them from don't so much prove that the effects are real (we already know that) — they explain why the effects happen. Conversely, the reality of the effects establishes the reality of the causes.
As for why I called them "hypotheses" — I did so specifically so people would know that while I believe I can derive them from the first truths I've already laid out, I've deliberately chosen not to do so here. I wanted to prevent a certain kind of person from seizing on what they take to be my principles and building some wild philosophical system on top of them — and then blaming me for it. I'm talking about people who think they can master in a single day what another person has spent twenty years thinking through, as soon as they've heard two or three sentences about it. Or people who are actually more error-prone and less capable of seeing the truth precisely because they're so clever and quick.
As for the ideas that are truly and entirely my own, I make no apology for their novelty. I'm convinced that if their reasons are carefully considered, they'll be found so simple and so aligned with common sense that they'll seem less strange and less paradoxical than any other positions that could be held on the same subjects. And I don't even claim to be the first person to come up with any of them — only that I've adopted them neither because other people held them nor because they didn't, but solely because reason convinced me they were true.
Even if skilled craftspeople can't immediately build the invention described in the Dioptrics, I don't think that's grounds for dismissing it. Building and adjusting the machines I describe requires practice and skill, and you can't afford to overlook the smallest detail. I'd be no less amazed if they succeeded on the first try than if someone became an accomplished guitar player in a single day just by looking at excellent sheet music.
And if I write in French, the language of my country, rather than Latin, the language of my teachers, it's because I expect that people who rely on their own unprejudiced natural reason will be better judges of my ideas than people who only pay attention to the writings of the ancients. As for those who combine good sense with serious study — the only judges I truly want — I'm confident they won't be so attached to Latin that they'll refuse to consider my arguments just because I've written them in everyday language.
In conclusion, I don't want to make any specific promises here about the progress I expect to make in the future, or to commit publicly to anything I'm not certain I can deliver. I'll say only this: I've resolved to spend whatever time I have left on nothing other than trying to gain the kind of knowledge of nature that will let us develop medical treatments more reliable than anything currently available. My temperament is so opposed to every other pursuit — especially any pursuit that can't help some people without hurting others — that if circumstances had forced me into such work, I don't think I could have succeeded.
I make this declaration publicly, fully aware that it won't win me any status in the world — which I don't care about in the least. And I will always feel more grateful to those who let me enjoy my quiet retirement without interruption than to anyone who might offer me the highest honors on earth.
To the Distinguished Faculty of Theology at the Sorbonne,
Gentlemen,
My reason for presenting this work to you is so straightforward that, once you understand what I'm trying to do, I'm confident you'll agree there's every reason for you to take it under your protection. The best way to recommend it to you is simply to explain what I set out to accomplish.
I've always believed that the two great questions — whether God exists and whether the soul is distinct from the body — are best settled by philosophy rather than theology alone. For those of us who already have faith, it's enough to accept on the authority of Scripture that the human soul doesn't die with the body and that God exists. But when it comes to persuading nonbelievers of the truth of any religion, or really even of basic morality, we first have to prove these two things using reason alone. After all, since this life so often rewards bad behavior more than good, very few people would choose to do the right thing over the profitable thing if they weren't motivated by either the fear of God or the hope of an afterlife.
Now, it's perfectly true that we should believe God exists because it's taught in Scripture, and we should believe Scripture because it comes from God. (Since faith is a gift from God, the same God who gives us the grace to believe other things can also give us the grace to believe in His own existence.) But you can't make this argument to nonbelievers — they'd rightly point out that it's circular reasoning.
And I've noticed that you, along with all the other theologians, have not only affirmed that natural reason is sufficient to prove God's existence, but also that Scripture itself implies our knowledge of God is clearer than our knowledge of many created things, and that this knowledge is so easy to come by that those who lack it are blameworthy. This is clear from the Book of Wisdom, chapter 13, where it says, "They are not to be excused; for if their understanding was so great that they could discern the world and its creatures, why did they not rather find out the Lord thereof?" And in Romans, chapter 1, it says they are "without excuse." In the same passage, the words "That which may be known of God is manifest in them" seem to tell us that everything we can know about God can be discovered through reasons found nowhere else but in our own minds. So I thought it would be entirely appropriate for me to investigate how God can be known — more easily and more certainly than the things of the world — simply by looking within ourselves, without going beyond our own minds.
As for the soul, although many have claimed that its nature is hard to discover, and some have even argued that human reason leads to the conclusion that the soul dies with the body (with only faith telling us otherwise), the Lateran Council held under Pope Leo X, in its eighth session, condemned these views. It expressly commanded Christian philosophers to refute such arguments and to establish the truth to the best of their ability. I have taken it upon myself to attempt this in this work.
On top of that, I'm aware that most irreligious people deny God's existence and the soul's distinctness from the body for no other reason than that, as they claim, no one has ever conclusively demonstrated these things. I completely disagree with them. On the contrary, I think that nearly all the proofs offered on these questions by great thinkers, when properly understood, carry the force of genuine demonstrations, and that it's practically impossible to come up with new ones. Still, I believe there's no more valuable service that could be performed in philosophy than for someone to carefully seek out the best of these arguments, once and for all, and present them so precisely and clearly that everyone would recognize them as true demonstrations from now on.
Finally, since many people who knew I had developed a certain method for solving all kinds of problems in the sciences were eager for me to apply it here — a method that isn't new in itself (since nothing is older than the truth), but one I had used successfully in other cases — I felt it was my duty to give it a try on this subject as well.
Everything I've been able to accomplish is contained in this work. I didn't try to gather every possible argument that could serve as proof on these topics — that didn't seem necessary, unless we were dealing with questions where no single proof of adequate certainty could be found. Instead, I took the most important arguments and developed them so thoroughly that I'd now venture to put them forward as demonstrations of the highest certainty and clarity. I'll even say that I believe no path is open to the human mind by which better proofs could ever be discovered. The importance of the subject and the glory of God — to which all of this relates — compel me to speak a bit more freely about my own work here than I usually do.
However, despite whatever certainty and clarity I may find in these demonstrations, I can't convince myself that they are accessible to everyone's understanding. It's the same as in geometry: there are many demonstrations by Archimedes, Apollonius, Pappus, and others that are accepted by everyone as certain and obvious — because they contain nothing that, taken on its own, is hard to understand, and no step that doesn't follow logically from the one before it. Yet very few people actually grasp them, because they are somewhat long and require the reader's full attention. In the same way, although I consider the demonstrations in this work to be equal to or even superior to geometrical proofs in certainty and clarity, I'm afraid many readers won't fully grasp them. They are somewhat long and involved, and, more importantly, they require a mind completely free from preconceptions and able to easily detach itself from the influence of the senses.
To be honest, the aptitude for metaphysical reasoning is less common than for geometry. Beyond that, there's another difference. In geometry, everyone is convinced that nothing is ever put forward without a solid proof, so people who only partially understand it more often make the mistake of accepting something false (wanting to seem like they understand it) than of rejecting something true. In philosophy, it's the opposite. Since people believe everything in philosophy is doubtful, very few sincerely devote themselves to searching for the truth. The vast majority prefer to build a reputation as bold thinkers by attacking truths that matter most.
This is why, however strong my arguments may be, I don't expect them to have much impact on people's minds — because they belong to the field of philosophy. Unless, that is, you lend them your support and approval. Your Faculty is held in such high esteem by everyone, and the name "Sorbonne" carries such authority, that no institution except the Sacred Councils receives greater deference — not only in matters of faith, but even in human philosophy. Everyone is convinced that there is no place where one can find greater intellectual rigor, more solid judgment, or more wisdom and integrity in reaching conclusions.
So I have no doubt that if you would be willing, first, to correct this work (since I'm well aware of both my human limitations and especially my ignorance, I don't claim it's free of errors); second, to fill in what's missing, to complete what's unfinished, and to provide fuller explanations where they're needed, or at least to point out these shortcomings so I can fix them; and third, once the arguments establishing God's existence and the distinction of the human soul from the body have been refined to the point where they can be recognized as rigorous demonstrations (which I'm confident they can be) — if you would then honor them with your official endorsement and publicly affirm their truth and certainty — I have no doubt, I say, that all the errors ever held on these questions would be quickly erased from people's minds.
The truth itself would naturally lead other thoughtful and educated people to accept your judgment. Your authority would cause the atheists — who are generally more pretentious than truly knowledgeable or intelligent — to drop their oppositional stance. It might even lead them to personally defend arguments that they see accepted as genuine demonstrations by all people of intellectual ability, rather than risk appearing unable to understand them. And finally, everyone else would readily trust so many testimonies, and there would no longer be anyone in the world who dared to doubt either God's existence or the real distinction between mind and body.
You, with your exceptional wisdom, are best positioned to judge how important it would be to establish these beliefs firmly — you who are well aware of the problems that doubt about these truths creates. But it wouldn't be appropriate for me to go on at greater length recommending the cause of God and religion to you, who have always been the strongest pillars of the Catholic Church.
I already touched briefly on the questions of God's existence and the nature of the human soul in my Discourse on the Method, published in French in 1637. But I didn't intend to treat those topics fully there — only to raise them in passing, so I could learn from my readers' reactions how best to handle them later. These questions seemed so important to me that they deserved more than a single treatment. And the path I follow in discussing them is so unfamiliar, so far from the usual approach, that I thought it wouldn't be wise to lay it all out at length in French, in a work that anyone might pick up. I was worried that weaker minds might think they could follow this path too.
In the Discourse, I had asked anyone who found something worth criticizing in my work to please let me know. And I can report that no objections worth mentioning were raised against what I said on these topics — except two. I'll reply to them briefly here, before taking them up in more detail later.
The first objection goes like this: even though the human mind, when it reflects on itself, doesn't perceive itself to be anything other than a thinking thing, it doesn't follow that its nature or essence consists only in being a thinking thing. The word "only" is the problem — it seems to rule out everything else that might also belong to the mind's nature. My reply is that I wasn't trying, in that passage, to rule things out according to how things actually are (which I wasn't dealing with at that point). I was only following the order of my own thinking. What I meant was simply this: as far as I was aware, I could clearly recognize nothing belonging to my essence except that I was a thinking thing — a thing with the ability to think. But I'll show later how, from the fact that I'm conscious of nothing else belonging to my essence besides thinking, it follows that nothing else really does belong to it.
The second objection is this: just because I have an idea of something more perfect than myself, it doesn't follow that the idea itself is more perfect than me, and even less that the thing the idea represents actually exists. My reply is that there's an ambiguity in the word "idea." It can be taken in two ways. In one sense, an idea is just an act of understanding — a mental event — and in that sense, it obviously can't be said to be more perfect than I am. But in another sense, "idea" refers to the thing represented by that mental act. And that thing, even if it doesn't exist outside my mind, can still be more perfect than me because of what it essentially is. I'll show more fully, later in this work, how the fact that I have an idea of something more perfect than myself proves that this thing really exists.
Beyond these two objections, I've also seen two rather lengthy essays attacking my work. But these attacked my conclusions far more than my premises, using arguments borrowed from the standard atheist playbook. Arguments like these can't make any impression on anyone who properly understands my reasoning. And since many people's judgment is so irrational and weak that they're more easily swayed by the first opinions they encounter on a subject — no matter how false or unreasonable — than by a true and solid refutation that comes afterward, I don't want to repeat those arguments here just to respond to them. I'd only be giving them their first hearing with some readers. I'll just say this in general: everything the atheists typically argue against God's existence always comes down to one of two mistakes. Either they project human qualities onto God, or they give our own minds too much credit, assuming we have enough power and wisdom to determine and understand what God can and should do. So nothing they argue will cause us any trouble, as long as we remember that our minds are finite while God is beyond our comprehension and infinite.
Now that I've tested the waters, so to speak, and gotten some sense of how people respond to my work, I'm taking up the topics of God and the human soul once again. At the same time, I'll be laying out the foundations of all of First Philosophy. I don't expect praise from the general public for this effort, and I'm not looking for a wide readership. In fact, I'd advise no one to read this work unless they're both able and willing to think seriously alongside me, to pull their minds away from the senses, and to free themselves from all preconceptions. People like that, I know very well, are remarkably rare. As for those who don't bother to follow the order and connection of my arguments, and instead just pick out isolated passages to nitpick — as many people do — I can say that they won't get much out of reading this. They may find opportunities to quibble here and there, but they're unlikely to raise any objections that are serious or worth responding to.
Since I certainly don't promise to satisfy everyone on all these matters at first glance, and I'm not so arrogant as to think I've anticipated every possible difficulty, here is my plan. First, in the Meditations themselves, I'll present the reasoning that has led me, I believe, to certain and evident knowledge of the truth, to see whether the arguments that convinced me can also convince others. Then I'll respond to the objections raised by several brilliant and learned people, to whom I sent these Meditations for criticism before publication. Their objections are so numerous and wide-ranging that I'm confident nothing important will easily come to mind that they haven't already addressed. So I strongly urge my readers: don't form any judgment about the questions raised in the Meditations until you've read through all of the Objections and Replies as well.
In the First Meditation, I lay out the reasons why we can doubt just about everything — especially the existence of physical objects — at least as long as we have no better foundations for the sciences than the ones we've relied on so far. Now, the usefulness of such extreme doubt might not be obvious at first. But it's actually enormously valuable, because it frees us from every prejudice, gives the mind the easiest possible path for pulling itself away from the senses, and ultimately makes it impossible for us to doubt whatever truths we discover afterward.
In the Second Meditation, the mind — exercising its own freedom — supposes that nothing it has even the slightest reason to doubt actually exists. But in doing so, it discovers that it itself must exist. This point is critically important, because it allows the mind to easily tell the difference between what belongs to it (that is, to its intellectual nature) and what belongs to the body.
Now, some readers might expect me to argue for the immortality of the soul at this stage. So let me explain why I don't. My aim was to write nothing that I couldn't demonstrate with precision. That meant I had to follow the same approach geometers use: establish all the building blocks a conclusion depends on before arriving at the conclusion itself.
The first and most important thing we need, in order to know that the soul is immortal, is to form the clearest possible conception of the soul itself — one that is completely distinct from every idea we have of the body. That's what the Second Meditation accomplishes.
Beyond that, we also need the assurance that everything we perceive clearly and distinctly is true — that things really are the way we clearly understand them to be. This can't be established until the Fourth Meditation.
We also need a clear understanding of physical nature, which is developed partly in the Second Meditation and partly in the Fifth and Sixth.
From all of this, we're finally able to conclude that things we clearly and distinctly understand to be different substances — like mind and body — really are distinct substances. That conclusion is drawn in the Sixth Meditation.
The absolute distinction between mind and body is also confirmed in the Second Meditation by showing that we can't conceive of a body without conceiving of it as divisible — but we can't conceive of a mind as divisible at all. You can imagine cutting any body in half, no matter how small. But you can't imagine half a mind. This tells us that these two natures are not just different, but in a real sense opposites.
I haven't pursued this argument all the way to its final conclusion in this work, for two reasons. First, even what I've said is enough to show that the destruction of the mind doesn't follow from the death of the body — and that's enough to give people hope in a future life. Second, fully proving the soul's immortality would require explaining the entire system of physics.
Here's why. We'd need to establish, first, that all substances — meaning everything that exists only because God created it — are by their very nature indestructible, and can never cease to exist unless God himself withdraws his sustaining power and reduces them to nothing. Second, we'd need to show that the body, taken in general terms, is a substance and therefore can never be destroyed. But the human body, specifically, is different from other bodies only because of how its parts are arranged and other features like that. The human mind, by contrast, isn't made up of such features. It's a pure substance.
To see the difference, consider this: even if everything about the mind changes — if it thinks different things, wills different things, perceives different things — the mind itself remains the same. But a human body stops being the same body when the arrangement of its parts changes. From this it follows that the body can easily perish, but the mind is by its very nature immortal.
In the Third Meditation, I've laid out my main argument for God's existence at what I think is sufficient length. However, since I wanted to avoid relying on comparisons drawn from physical objects — because I was trying to pull my readers' minds as far from the senses as possible — some parts may still seem unclear. These obscurities, I trust, will be fully cleared up in the Replies to the Objections.
For example, it might be hard to understand how the idea of a supremely perfect being, which we find in our minds, contains so much "objective reality" (that is, represents so high a degree of being and perfection) that it must come from a supremely perfect cause. This is illustrated in the Replies by the comparison of an incredibly sophisticated machine. The idea of this machine exists in some engineer's mind, and that idea's perfection must have a source — either the engineer's own knowledge, or the knowledge of someone who passed the idea on to the engineer. In the same way, the idea of God that we find in ourselves requires God himself as its cause.
In the Fourth Meditation, I show that everything we perceive clearly and distinctly is true. At the same time, I explain what error actually is. Both points need to be understood — both to confirm the truths established earlier and to make sense of what comes later.
One important clarification: in this Meditation, I'm not talking about sin — that is, errors in the pursuit of good and evil. I'm talking only about the kind of error that occurs when we're trying to determine what's true and what's false. And I'm not addressing matters of faith or how to live our lives. I'm concerned only with truths we can discover through our natural reason alone.
In the Fifth Meditation, beyond further explaining physical nature in general, I offer a new proof of God's existence. This proof may also have some difficulties — just as the earlier one did — but these will be addressed in the Replies to the Objections. I also show in what sense the certainty of geometric proofs themselves depends on our knowledge of God.
Finally, in the Sixth Meditation, I distinguish the act of understanding from the act of imagining and describe the differences between them. I show that the human mind is truly distinct from the body, and yet is so closely joined to it that the two together form something like a single unit. I review all the errors that come from the senses and show how to avoid them. And I present all the reasons from which we can infer that physical objects exist.
I include these arguments not because I think they're especially important for proving what they prove — that the world is real, that we have bodies, and things like that. No one in their right mind has ever seriously doubted those things. I include them because, when you examine them carefully, you realize they're not nearly as strong or clear as the reasoning that leads us to knowledge of our own minds and of God. This means that knowledge of the mind and of God is the most certain and evident of all human knowledge. Establishing this single conclusion was my entire aim in these Meditations, which is why I'm leaving aside the various other questions I touched on along the way.
It's been years now since I first realized something unsettling: a lot of what I'd accepted as true since childhood was actually false. And everything I'd built on those shaky foundations — my whole understanding of the world — was therefore deeply unreliable. From that moment, I knew that at some point in my life, I would have to tear everything down and start over from scratch, if I ever wanted to build anything solid in the sciences. But this seemed like such an enormous project that I kept putting it off, waiting until I was old enough that I could be sure no future version of me would be better equipped for the job. Well, I've waited long enough. If I keep stalling now, I'll be wasting whatever time I have left.
So: today is the day. I've cleared my schedule. My mind is free from distractions, undisturbed by any strong emotions. I have the peace and quiet I need. Today, finally, I'm going to systematically demolish everything I believe.
Now, to do this, I don't need to prove that every single one of my beliefs is false — that might be impossible anyway. But here's the key insight: my reason already tells me I should withhold belief from things that aren't completely certain and beyond doubt, just as carefully as I would from things that are obviously false. So if I can find any reason to doubt a belief, no matter how small, that's enough to reject it — at least for now. And I don't need to go through my beliefs one by one, which would take forever. Instead, I'll go straight to the foundations. If I can knock out the principles at the base, everything built on top of them will collapse on its own.
So what are those foundations? Everything I've ever accepted as most true and certain came to me either directly through the senses, or from reasoning based on what the senses told me. But I've noticed that the senses sometimes deceive us. And it's simply good sense never to place complete trust in something that has tricked you before, even once.
Now, someone might object: "Sure, the senses can fool us about small things, or objects that are very far away. But there are plenty of things we perceive that clearly can't be doubted. For instance, right now I'm here, sitting by the fire, wearing my winter robe. I'm holding this paper in my hands. How could I possibly deny that these hands, this body, are mine?"
And honestly, how could I deny it — without making myself sound like a lunatic? After all, there are people who are so deranged that they believe they are kings when they are beggars, or that they are dressed in gold and purple when they are naked, or that their heads are made of clay, or their bodies of glass, or that they are literally gourds. But those people are insane. And I would be just as insane if I took their example as a guide.
Fair enough. But here's the thing: I'm a human being. And human beings sleep. And when we sleep, we dream — and in our dreams, we experience things just as vivid as what those deranged people experience while awake, or even more so.
Think about it. How many times have I dreamed that I was right here, in this very room, sitting by this fire, wearing these clothes — when in reality I was lying undressed in my bed the whole time?
Right now, of course, I'm wide awake. Aren't I? I'm looking at this paper with open eyes. This head that I'm moving is not asleep. I reach out my hand deliberately, consciously, and I feel it. None of this is as vague and blurry as a dream.
But wait. I've been fooled like this before. I've had dreams where I was convinced I was awake, where everything felt just as real and sharp as this moment feels right now. And when I think about it carefully — really carefully — I realize something disturbing: there is no reliable test, no sure sign, that can definitively tell me whether I'm awake right now or dreaming. The realization is dizzying. I'm almost ready to convince myself that I might, right now, be asleep.
Alright then. Let's suppose I am dreaming. Let's say that none of what I'm experiencing is real — that opening my eyes, moving my head, reaching out my hands, all of it is just illusion. Let's even suppose that I don't really have a body or hands at all.
But even in a dream, the images have to come from somewhere. They're like paintings. And even the most fantastical painting is made up of elements drawn from reality. When painters create imaginary creatures — sirens, satyrs, monsters — they don't invent entirely new forms. They just recombine parts of real animals in strange ways. And even if a painter somehow conjured up something so utterly original that nothing like it had ever existed before, at the very least the colors in the painting would have to be real.
By the same logic, even if everything I'm experiencing in this dream — my body, my eyes, my head, my hands — is imaginary, there must be some simpler, more basic ingredients that are real. These would be like the real colors that even the most fantastical painting is made from. The images in our minds, whether accurate or totally fabricated, have to be built out of something.
These basic ingredients seem to include things like physical nature in general, along with extension — the fact that things take up space. They include shape, size, number, and also place and time. These are the most fundamental building blocks.
And this leads to an interesting conclusion. Sciences that deal with complex, real-world objects — physics, astronomy, medicine, and the like — suddenly look very uncertain. But mathematics? Arithmetic, geometry, and similar fields? These deal with the simplest, most basic objects, and they don't even care much about whether those objects actually exist in the physical world. Whether I'm awake or dreaming, two plus three still equals five. A square still has four sides. It seems like truths this elementary could never be in doubt.
And yet...
There's something I've believed for a long time: that there is a God, and that this God is all-powerful, and that he made me. Well then, how do I know he hasn't arranged things so that there is no earth, no sky, no physical world at all — while still making me experience all of it as though it were real? How do I know he hasn't set things up so that I'm wrong every single time I add two and three, or count the sides of a square, or perform any operation however simple?
But maybe God wouldn't do that. After all, he's supposed to be supremely good. Then again, if it goes against his goodness to create me so that I'm constantly deceived, it should also go against his goodness to let me be deceived even occasionally. And yet — here I am, sometimes getting things wrong. Clearly, he does allow it.
Some people, confronted with this line of thinking, might prefer to deny that such a powerful being exists at all, rather than accept that nothing is certain. But let's set that aside and grant, for the sake of argument, that everything I've said about God is just a story. It doesn't help. However I ended up existing — whether by fate, by accident, by some endless chain of causes, or by any other means — the less powerful and intelligent my origin, the more likely it is that I'm so imperfect as to be constantly fooled. I have no good reply to this.
So I'm forced to admit that there is nothing in all my former beliefs that is beyond doubt. And this isn't some careless or flippant conclusion. It's the result of careful, deliberate reasoning. Going forward, if I want to find something genuinely certain, I need to withhold my assent from these old beliefs just as firmly as I would from something obviously false.
But recognizing this isn't enough. I also need to keep reminding myself of it, because those old opinions don't just vanish. They keep coming back. Years and years of habit have given them squatter's rights in my mind. They occupy my thoughts almost against my will, and I can't stop trusting them, not as long as I see them for what they actually are: opinions that are somewhat doubtful, yes, as I've just shown, but still quite plausible — far more reasonable to believe than to deny.
That's exactly why I think the right move is to deliberately deceive myself for a while. I'll pretend, on purpose, that all these old beliefs are completely false and imaginary. I'll keep this up until my new, skeptical perspective has enough weight to balance out the old habits, and my judgment is no longer pulled off course by years of uncritical thinking. I know this can't do any harm, because what I'm after right now isn't action but knowledge. I can't be too skeptical for my present purposes.
So here is what I will suppose. Not that God, who is perfectly good and the source of all truth, is deceiving me. Instead, I will imagine that there is an evil genius — a being of supreme power and cunning — who has devoted all of his energy to tricking me. I will suppose that the sky, the air, the earth, colors, shapes, sounds, and every external thing are nothing but illusions, the deceptive stage set of this evil genius, designed to ensnare my mind. I will consider myself as having no hands, no eyes, no flesh, no blood, no senses at all — and as merely believing, falsely, that I have all of these. I will hold to this belief with grim determination. And even if I can't manage to reach the truth this way, I will at least do this much: refuse to accept anything false. I will guard myself, with everything I have, against this deceiver, however powerful and clever he may be.
But this is harder than it sounds. A kind of laziness keeps pulling me back toward my ordinary life. I'm like a prisoner who was dreaming of freedom. When he starts to suspect that the freedom was only a dream, he doesn't want to wake up. He clings to the pleasant illusion, afraid of what waking will bring. In the same way, I keep sliding back into my old comfortable beliefs, afraid to shake myself fully awake — afraid that after the peaceful sleep of ignorance, what comes next won't be the light of day, but just a deeper darkness born from all these doubts I've stirred up.
Yesterday's meditation has filled my mind with so many doubts that I can no longer push them aside. And I don't see any way to resolve them. It's as if I suddenly fell into deep water — I'm so disoriented that I can't plant my feet on the bottom or keep myself afloat at the surface. But I'm going to push forward anyway, picking up the same path I started on yesterday. I'll proceed by throwing out everything that allows even the slightest doubt — treating it as if I'd proven it absolutely false — and I'll keep going until I find something certain. Or at the very least, until I know for certain that nothing is certain. Archimedes said he could move the entire earth if he had just one fixed and immovable point to stand on. In the same way, I'll have reason for great hope if I can discover just one thing that is certain and beyond all doubt.
So here's where I stand. I'm assuming that everything I see is an illusion. I believe that nothing my unreliable memory presents to me ever actually existed. I'm assuming I have no senses at all. Body, shape, extension, motion, location — I believe all of these are just inventions of my mind. What, then, could possibly be true? Maybe only this: that absolutely nothing is certain.
But wait. How do I know there isn't something entirely different from everything I've just listed — something that I can't doubt at all? Is there a God, or some being (call it whatever you like) who puts these thoughts into my mind? But then again, why assume there's some external being doing this? Maybe I'm producing these thoughts myself. But then — am I at least something? Hold on, though. I already denied that I have a body or senses. But what follows from that? Am I really so tied to my body and senses that I can't exist without them?
Well, I had convinced myself that there was absolutely nothing in the world — no sky, no earth, no minds, no bodies. So didn't I also convince myself that I didn't exist? No — absolutely not. If I was convinced of something, then I most certainly existed. But there is some being — I don't know what it is — supremely powerful and supremely cunning, who is using all of its ingenuity to deceive me at every turn. Then there is no doubt: I exist, if this being is deceiving me. And let it deceive me as much as it can — it can never make me into nothing, as long as I'm aware that I am something. So after thinking it through as carefully and thoroughly as I possibly can, I have to conclude that this statement — I am, I exist — is necessarily true every single time I say it or think it.
But I still don't know clearly enough what I am, this "I" that I now know exists. So I need to be careful. I don't want to make the mistake of confusing myself with something else and going astray on what should be the most certain piece of knowledge I have. To avoid that, I'll reconsider what I used to believe about myself, before I started down this path. I'll strip away everything that could be undermined — even slightly — by the doubts I raised yesterday, until nothing remains but what is absolutely certain.
So what did I used to think I was? I thought I was a human being, of course. But what is a human being? Should I say "a rational animal"? No — that would immediately require me to define "animal" and "rational," and one question would lead to another, each harder than the last. I don't have time to waste on those kinds of technicalities. Instead, let me focus on what naturally came to mind when I used to think about what I was, without any philosophical framework.
First, I thought I had a face, hands, arms, and the whole structure of limbs that you see in a body — what I called "my body." Beyond that, I thought I was nourished, that I moved around, that I perceived things, and that I thought — and I attributed all of these activities to my soul. As for what the soul actually was, I either didn't think about it or, if I did, I vaguely imagined it as something incredibly fine and subtle — like a breath of air, or a flame, or a thin vapor — spread through my physical parts. About my body, though, I had no doubts at all. I was sure I knew exactly what it was. If you'd asked me to describe it at the time, I would have said something like this: a body is anything that has a definite shape; that can be located in a specific place, filling that space so no other body can be there at the same time; that can be perceived by touch, sight, hearing, taste, or smell; and that can be moved in various ways — not by itself, but by whatever pushes or touches it. I didn't think the ability to move on its own, or to perceive, or to think, had anything to do with the nature of a body. In fact, I was a bit surprised to find these abilities in some bodies.
But now I'm supposing there's an extremely powerful and — if I can put it this way — malicious being whose entire effort is devoted to deceiving me. So can I say I have any of the features I just described as belonging to a body? I think about it carefully, go through them one by one, and... no. None of them can properly be said to belong to me. There's no point running through the list again.
What about the attributes of the soul, then? The first ones were nutrition and walking. But if I really don't have a body, I can't eat or walk. Next is perception — but perception is impossible without a body too. And besides, I've often believed in my sleep that I was perceiving things that I later realized I wasn't perceiving at all.
Then there's thinking. And here — here I find something. Thinking is the one attribute that can't be separated from me. I am — I exist. That much is certain. But for how long? For exactly as long as I'm thinking. Because if I were to stop thinking entirely, it's possible I would stop existing entirely. Right now I'm admitting only what has to be true. Strictly speaking, then, I am only a thinking thing — that is, a mind, an understanding, a reason. (These are terms whose meaning I didn't truly grasp before.) But I am something real, something that really exists. What kind of thing? I just said it: a thinking thing.
So the next question is: am I anything more than that? Let me try using my imagination to see if I'm also something beyond a thinking being. Well, it's clear I'm not the collection of limbs called a human body. I'm not some thin, invisible gas spread through those limbs. I'm not wind, fire, vapor, or breath, or anything else I can dream up — because I already assumed none of those things exist, and even with that assumption firmly in place, I still know with certainty that I exist.
But here's a possibility: maybe those very things I'm dismissing as nonexistent — things unknown to me — are actually not different from the "I" that I do know. I can't settle that question, and I'm not going to argue about it now. I can only make judgments about things I already know. I know I exist. And I'm asking what this "I" is that I know. It's absolutely certain that my knowledge of myself, taken in this precise way, doesn't depend on anything whose existence I haven't yet established. And it definitely doesn't depend on anything I can construct in my imagination.
In fact, that phrase — "construct in my imagination" — reveals the problem. Imagining is really just forming a mental picture of some physical thing. But I already know I exist, and I know it's possible that all these mental images, and everything related to physical bodies, are nothing more than dreams. Given that, it would be absurd to say, "Let me use my imagination to figure out more clearly what I am." That would be like saying, "I'm awake right now and I can perceive something real, but my perception isn't clear enough — so let me deliberately fall asleep, because maybe my dreams will give me a truer and clearer picture." No — I can see that nothing I grasp through imagination belongs to my knowledge of myself. I need to carefully pull my mind back from that way of thinking, so it can recognize its own nature with perfect clarity.
So what am I, then? A thinking thing. And what is that? It's a thing that doubts, understands, affirms, denies, is willing, is unwilling, and also imagines and perceives through the senses.
That's actually quite a lot, if all of these belong to my nature. But why wouldn't they? Am I not the very being who right now doubts almost everything, yet still understands certain things? Who affirms this one truth and denies all the rest? Who wants to know more and doesn't want to be deceived? Who imagines many things — sometimes even against my will — and who perceives many things, as though through the senses?
Is any of this less true than the fact that I exist — even if I'm always dreaming, even if the being who created me is doing everything in its power to trick me? Is any of these activities really separable from my thinking, or distinguishable from myself? It's so obvious that it's I who doubt, I who understand, and I who will, that there's nothing I could say to make it any clearer. And I am just as certainly the being who imagines. Even if none of the things I imagine are real (as I supposed before), the power of imagination itself is real and is part of my thinking. And I am the same being who perceives — who seems to see light, hear sounds, and feel heat. Someone might object: "Those are false impressions. You're dreaming." Fine, let it be so. But it's still certain that I seem to see light, seem to hear sounds, and seem to feel heat. That much cannot be false. And this — this seeming, this awareness — is precisely what is called perception. Perception, understood in this strict sense, is nothing other than thinking.
From all this, I'm starting to understand what I am with more clarity than before. And yet, I have to admit, I can't help feeling that physical things — the things whose images are formed in my mind, the things I can examine with my senses — are somehow known more vividly than this mysterious part of myself that can't be pictured in the imagination. It does seem strange that I'd know things whose existence I've called into question — things that are unfamiliar to me and don't belong to me — more clearly than I know myself, something whose reality I'm certain of. But I can see what's happening. My mind loves to wander and won't stay within the boundaries of truth. So let me give it some freedom. Let me let it turn to the objects of the external world for a while, so that later, when I gently rein it back in and direct it toward its own nature, it will be easier to control.
Let me consider, then, the things that people ordinarily think are the easiest and most clearly understood: namely, the physical things we touch and see. Not bodies in general — general concepts tend to be vaguer. Let me take one specific thing.
Consider this piece of wax. It was just taken from the beehive. It hasn't yet lost the sweetness of the honey it contained. It still carries a faint scent of the flowers the honey was gathered from. Its color, shape, and size are all plain to see. It's hard. It's cold. You can pick it up and handle it easily. Tap it with your finger and it makes a sound. Everything that could possibly help you know a physical object as clearly as you can — all of it is right here in this piece of wax.
But now — while I'm still talking — let me hold it near the fire.
The last traces of flavor disappear. The scent evaporates. The color changes. The shape is lost. It gets bigger. It turns to liquid. It grows hot. I can barely hold it anymore. And if I tap it, it makes no sound.
Is it still the same wax? Obviously it is. No one would deny it. No one would think otherwise.
Then what was it about the wax that I knew so clearly before? It can't have been anything I detected through the senses — because every quality I saw, smelled, tasted, touched, and heard has changed. And yet the same wax remains.
Maybe what I knew all along was this: the wax was never really the sweetness, the pleasant smell, the white color, the shape, or the sound. It was simply a body that appeared to me first in one set of forms and now appears in others. But what exactly am I thinking when I conceive of it this way? Let me strip away everything that doesn't truly belong to the wax and see what's left.
What's left is just this: something extended, flexible, and changeable. But what do I mean by "flexible and changeable"? Am I just picturing in my mind that the wax, which is now round, could become square, or change from a square into a triangle? No — that can't be it. Because I know the wax can take on an infinite number of shapes, and my imagination can't run through an infinite number of anything. So my understanding of the wax isn't a product of imagination.
And what about "extended" — what do I really mean by that? The wax's extension is also unknown in a way, because it gets larger when the wax melts, larger still when it boils, and even larger as the heat increases. I wouldn't be conceiving of the wax accurately if I didn't think it could take on far more variations in size than I could ever picture in my mind. So I have to admit it: I can't even comprehend what this piece of wax is through my imagination alone. Only my mind can perceive it.
And I'm talking about this one particular piece of wax — because the point is even more obvious if we're talking about wax in general. So what is this piece of wax that can only be grasped by the mind? It's the very same wax I see, touch, and imagine. It's the same wax I believed it to be from the beginning. But here's the crucial point: my perception of the wax is not, and never was, an act of seeing, touching, or imagining — even though it used to seem that way. It's an act of the mind alone. It might be a confused and imperfect act of the mind, as it was before, or a clear and distinct one, as it is now. The difference depends entirely on how carefully I attend to what the wax is made of.
And yet I'm astonished when I notice how prone my mind is to error. Even now, as I'm thinking all this through silently, I keep getting tripped up by ordinary language. We say we "see" the same wax when it's in front of us, rather than saying we judge it to be the same because it looks the same — which might lead me to conclude that I know the wax through sight, not through the mind alone.
But then I happen to glance out the window at people walking by on the street below. And I naturally say, "I see people." But what do I actually see? Hats and coats. Underneath those hats and coats, there could be robots controlled by springs and gears, for all I know. But I judge that they're real people. So what I thought I was seeing with my eyes, I was actually grasping with the faculty of judgment that exists only in my mind.
A person who aims to achieve knowledge beyond ordinary understanding shouldn't be looking for reasons to doubt based on common ways of speaking. So let me move on and ask: did I perceive the wax more clearly and perfectly when I first laid eyes on it and thought I knew it through my external senses, or at least through what's called "common sense" — the faculty of imagination? Or do I understand it more clearly now, after I've examined more carefully both what it is and how it can be known? It would be absurd to hesitate on this question. What was there in my first perception that was truly distinct? What did I perceive that any animal couldn't have perceived just as well? But when I separate the wax from its outward appearances — when I mentally strip away its coverings and consider it naked, as it really is — then even though my judgment might still contain some error, I certainly can't understand it this way without a human mind.
But finally — and this is the real payoff — what should I say about the mind itself? About myself? Because so far I'm maintaining that I am nothing but a mind. Here I am, seeming to have such a clear grasp of this piece of wax. But don't I know myself even more truly, more certainly, more clearly and distinctly than I know the wax?
Think about it. If I judge that the wax exists because I see it, then it follows far more obviously that I myself exist, because I'm the one seeing. It's possible that what I see isn't really wax. It's even possible that I don't have eyes at all. But it cannot be the case that when I see — or, what amounts to the same thing, when I think I see — that I, the one who is thinking, am nothing.
Similarly, if I judge that the wax exists because I touch it, the same conclusion follows: I exist. If I judge that the wax exists because I imagine it, or for any other reason at all — the same conclusion follows every time. And what I've just said about the wax applies to every single thing in the external world.
What's more, if my perception of the wax seemed to become sharper and more distinct as I examined it through more and more means — not just sight and touch, but many other approaches as well — then how much more clearly must I know myself? Because every single reason that helps me understand the wax, or any physical object, does an even better job of revealing the nature of my own mind. And the mind has so many other qualities that illuminate its nature that the ones depending on the body barely deserve to be mentioned.
And so I find I've arrived, almost without noticing, exactly where I wanted to be. It's now clear to me that physical objects aren't really perceived by the senses or the imagination, but by the intellect alone. And they're not perceived because they're seen or touched, but because they're understood by thought. From this I easily see that nothing is more readily or clearly grasped than my own mind.
But because it's hard to let go of a belief you've held for a long time, I want to pause here. Let me sit with this new understanding a while, so that through extended reflection, I can impress this truth more deeply on my memory.
I'm now going to close my eyes, stop my ears, and shut out all my senses. I'll even try to erase from my mind every image of physical things — or at least, since that's nearly impossible, I'll treat those images as empty and meaningless. Holding a conversation only with myself and looking carefully at my own nature, I'll try to build, step by step, a deeper and more familiar understanding of who I am.
I am a thinking, conscious thing — a being that doubts, affirms, denies, knows a few things and is ignorant of many, that loves, hates, wills, refuses, that imagines and perceives. As I noted before, even if the things I perceive or imagine have no reality outside of me, I can still be sure that those ways of being conscious — what I'm calling perceptions and imaginings — do exist in me, as aspects of my thought.
In what I've just said, I think I've summed up everything I truly know — or at least everything I was aware of knowing up to this point. Now I want to push further and look carefully to see if there's anything else in myself that I haven't noticed yet. I'm certain that I am a thinking thing. But does that tell me what's actually required for me to be certain of any truth? In this first piece of knowledge, the only thing that gives me assurance is the clear and distinct perception of what I'm affirming. That wouldn't be enough to guarantee truth if it were possible for something I clearly and distinctly perceived to turn out to be false. So it seems I can now adopt a general rule: everything I perceive very clearly and distinctly is true.
And yet, I've previously accepted many things as absolutely certain that I later found to be doubtful. What were those things? The earth, the sky, the stars, and all the other objects I used to perceive through the senses. But what did I actually perceive clearly about them? Nothing more than that ideas and thoughts of those objects were present in my mind. Even now I don't deny that those ideas exist in me. But there was something else I used to affirm — something I believed I clearly perceived, though in truth I didn't perceive it at all. I mean the existence of external objects from which those ideas supposedly came and which those ideas supposedly resembled perfectly. That's where I went wrong. Or if I happened to judge correctly on some point, it wasn't because of any genuine knowledge I possessed.
But what about when I considered something in arithmetic or geometry — something very simple and straightforward, like the fact that two plus three equals five? Didn't I see that clearly enough to affirm it was true? The only reason I later thought we should doubt such things was this: it occurred to me that perhaps God gave me a nature that makes me prone to deception, even about things that seem most obviously true. But whenever this worry about God's supreme power comes to mind, I have to admit it would be easy for him — if he chose — to make me go wrong even about things where I think I have the strongest evidence. On the other hand, whenever I focus on things I think I grasp with perfect clarity, I'm so completely convinced of their truth that I can't help but say: Let anyone deceive me who can — they'll never bring it about that I don't exist, as long as I'm conscious that I do exist. They'll never make it true at some future time that I have never existed, when it's true right now that I do exist. They'll never make two and three anything other than five. Contradictions like that are impossible.
And in fact, since I have no reason to think God is a deceiver — and since I haven't even established yet whether God exists — this source of doubt, which depends entirely on that assumption, is pretty thin. It's metaphysical, you might say. But to remove it completely, I need to investigate whether God exists as soon as the opportunity arises. And if I find that God does exist, I also need to determine whether he could be a deceiver. Without knowing these two things, I don't see how I can ever be certain of anything.
Now, so that I can pursue this inquiry without breaking the order I've set for myself — which is to move step by step from what I find first in my mind to what I discover later — I need to sort my thoughts into categories and figure out which ones are capable of truth and error in the strict sense.
Some of my thoughts are like images of things, and it's only these that properly deserve the name "idea" — for instance, when I think of a person, a mythical creature, the sky, an angel, or God. Other thoughts have additional forms beyond mere representation: when I will something, fear something, affirm or deny something, I always have some object in mind, but my thought also includes something beyond the mere image of that object. Thoughts in this second category include acts of will, emotions, and judgments.
Now, when it comes to ideas considered purely in themselves — not as pointing to anything beyond themselves — they can't, strictly speaking, be false. Whether I imagine a goat or a chimera, it's equally true that I'm imagining one or the other. Nor do we need to worry about falsity in the will or emotions. I might desire things that are wrong or that don't even exist, but it's still true that I desire them. So the only place where I need to watch out for error is in my judgments. And the most common error in judgment is this: I assume that the ideas in my mind resemble things outside of me. If I just considered my ideas as certain ways of thinking — without assuming they correspond to external objects — they'd hardly give me any opportunity to go wrong.
Among my ideas, some seem to be innate, others seem to come from outside me, and still others seem to be made up by me. My ability to conceive of a thing, a truth, or a thought — that seems to come from my own nature and nothing else. But if I hear a noise right now, or see the sun, or feel warmth, I've always judged that those sensations come from objects outside of me. And finally, things like sirens, griffins, and the like are clearly inventions of my own mind. That said, I might end up concluding that all my ideas are really of the externally-caused type, or all innate, or all invented. I haven't yet clearly pinpointed their true origin.
What I mainly need to do right now is examine those ideas that seem to come from external objects and ask: what reason do I actually have for thinking they resemble those objects? There seem to be two reasons. First, nature seems to teach me so. Second, I notice that these ideas don't depend on my will and therefore don't depend on me — they often come to me against my will. Right now, for example, whether I want to or not, I feel heat. So I'm naturally inclined to think this sensation of heat is produced in me by something other than myself — namely, the heat of the fire I'm sitting by. And it seems perfectly reasonable to suppose that the fire impresses its own likeness on me, rather than something else entirely.
But I need to ask whether these reasons are really strong enough.
When I say "nature teaches me," I only mean a kind of spontaneous impulse that pushes me to believe my ideas resemble their objects. I don't mean the "natural light" — that is, genuine rational insight — which actually gives me knowledge of truth. These two things are very different. What the natural light reveals to be true is beyond doubt — for example, that I exist because I doubt, and similar truths. I have no other faculty for distinguishing truth from error that could overrule the natural light, and the natural light is completely trustworthy. But as for those spontaneous impulses? I've noticed many times, when it came to choosing between right and wrong in action, that they often led me astray. So I have no reason to trust them on questions of truth and error either.
As for the other reason — that these ideas don't depend on my will and must therefore come from external objects — I don't find it any more convincing. Just as those natural impulses exist in me even though they don't always match my will, it's possible that I have some faculty I'm not fully aware of that produces ideas on its own, without any help from external objects. After all, that seems to be exactly what happens when I dream: ideas are produced in me by some internal power, without any external cause.
And finally, even if I granted that my ideas do come from external objects, it doesn't follow that they must resemble those objects. In fact, I've noticed many cases where there's a huge difference between an object and the idea of it. Take the sun, for example. I find two completely different ideas of the sun in my mind. One comes from my senses: the sun appears tiny, barely a speck in the sky. The other is based on astronomical reasoning: the sun is many times larger than the entire Earth. Both of these ideas can't accurately resemble the same sun, and reason tells me the one that seems to come directly from sense experience is actually the least accurate representation.
All of this shows clearly enough that, until now, my belief in external objects hasn't been based on careful, deliberate judgment. It's been based on a kind of blind impulse.
But there's another way to approach this question: is there anything among my ideas whose cause must lie outside of me?
If I consider my ideas merely as ways of being conscious, I don't see any difference or inequality among them — they all seem to come from me in the same way. But when I consider them as representations — as mental images where one depicts one thing and another depicts something different — then there are obviously huge differences between them. Ideas that represent substances clearly contain more, so to speak, than ideas that represent mere properties or features of substances. And the idea by which I conceive of God — a supreme, eternal, infinite, unchangeable, all-knowing, all-powerful being who created everything else that exists — this idea certainly contains more representational content than the ideas of finite things.
Let me explain what I mean by "representational content," because this is important. Every idea represents something — it has content. The degree of reality or perfection of what an idea represents, I'll call its "objective reality." (This is a technical term, and it doesn't mean what "objective" usually means in modern speech. It refers to the reality of the content of the idea — the thing the idea is about, as it exists within the idea.) And "formal reality" is the degree of reality something has in itself, as an actually existing thing. The key principle I want to establish is about the relationship between these two.
Now, it's obvious by the natural light of reason that there must be at least as much reality in a cause as in its effect. Where else could the effect get its reality, if not from its cause? And how could the cause give it that reality without possessing it? From this it follows that something can't come from nothing, and also that what is more perfect — what contains more reality — can't be produced by what is less perfect.
This isn't just true for things that actually exist in the world. It's also true for ideas and their representational content. A stone that doesn't yet exist can't begin to exist unless it's produced by something that already contains, either straightforwardly or in some higher form, everything that makes up the stone. Heat can only be produced in something by a cause that is at least as perfect as heat. And the same goes for ideas: the idea of heat, or the idea of a stone, can't exist in my mind unless it was put there by a cause that contains at least as much reality as I conceive to be in the heat or the stone.
Now, it's true that an idea, as a thing in itself, is just a mode of thought — its existence as a mental event doesn't require much. But in order for an idea to have this particular representational content rather than that one, it must get that content from some cause that has at least as much actual reality as the idea has representational content. If you suppose an idea contains something that wasn't in its cause, then that something would have to come from nothing. But however modest the kind of existence something has when it exists merely as the content of an idea in someone's mind, that existence isn't nothing. So the idea can't come from nothing either.
And I shouldn't think that just because the reality I'm considering in my ideas is merely representational, the cause of that reality only needs to have representational reality too. No — just as the representational mode of existence belongs to ideas by their very nature, the actual mode of existence belongs to the causes of ideas by their very nature (at least to the first and fundamental cause). And while one idea can give rise to another, this chain can't go on forever. Eventually we have to arrive at a first idea whose cause is like an original blueprint — something that actually and formally contains all the reality or perfection that the ideas only contain representationally.
So the natural light teaches me this: ideas exist in me like pictures or images. These pictures may fall short of the things they represent, but they can never contain anything greater or more perfect than those things' actual source.
The more carefully I think about all this, the more clearly I see it. But let me draw the practical conclusion. It's this: if any one of my ideas has so much representational content that I'm sure that same reality doesn't exist in me — either actually or in some higher form — then I myself can't be the cause of that idea. And if I can't be its cause, it necessarily follows that I'm not alone in the world. Some other being must exist as the cause of that idea. On the other hand, if I can't find any such idea, then I have no solid reason for believing anything else exists besides myself. And believe me, I've searched very carefully.
So let me go through my ideas. Setting aside the idea that represents me (which presents no difficulty here), there is an idea that represents God. Then there are ideas of physical, lifeless things; ideas of angels; ideas of animals; and ideas of other people like myself.
The ideas of other people, animals, and angels — I can easily see how these could have been assembled from the other ideas I already have: ideas of myself, of physical things, and of God. Even if no other people, animals, or angels existed in the world, I could have put these ideas together on my own.
As for ideas of physical objects — I can't find anything in them so great or impressive that it seems beyond my own ability to produce. If I look at them closely and examine them one by one, the way I examined the idea of wax yesterday, I find there's very little in them that I perceive clearly and distinctly. What I do clearly grasp includes: size (that is, extension in length, width, and depth), shape (which results from the boundaries of that extension), the spatial relationships between differently shaped objects, and motion (the change of those relationships). To these I can add substance, duration, and number.
But as for light, colors, sounds, smells, tastes, heat, cold, and the other qualities we perceive by touch — I think of these only with such obscurity and confusion that I can't even tell whether they're real or illusory. In other words, I don't know whether my ideas of these qualities are ideas of real things or ideas of nothing at all. I mentioned earlier that formal falsity — falsity in the strict sense — can only occur in judgments. But there's a kind of material falsity in ideas themselves: it happens when an idea represents something that is nothing as if it were something. For example, my ideas of heat and cold are so far from being clear and distinct that I can't tell from them whether cold is just the absence of heat, or heat the absence of cold, or whether both are real qualities in their own right. And since ideas are like images — each one seeming to represent some object — if cold is really nothing but the absence of heat, then an idea that represents cold as something real and positive would, in a sense, be false.
For ideas like these, I don't need to look for any source beyond myself. If they're false — if they represent things that don't exist — then the natural light tells me they arise from some deficiency in my nature, from nothing, in a manner of speaking. And if they happen to be true, they show me so little reality that I can't even distinguish what they represent from nonexistence. So I see no reason why I couldn't be their author.
As for those ideas of physical things that are clear and distinct, there are some that I could seemingly have derived from the idea I have of myself: substance, duration, number, and so on. When I think that a stone is a substance — something capable of existing on its own — and that I too am a substance, I notice that these are very different concepts (I'm a thinking, non-physical thing, while the stone is a physical, non-thinking thing). Yet both ideas share the concept of substance. Similarly, when I think of myself as existing now while also remembering that I existed in the past, or when I'm aware of having various thoughts that I can count, I acquire the ideas of duration and number, which I can then apply to other things. As for extension, shape, spatial position, and motion — since I'm nothing but a thinking thing, these qualities aren't formally in me. But because they're just particular ways a substance can be, and because I myself am a substance, it seems possible that I contain them in some higher or more fundamental way.
So there remains only the idea of God, and I need to consider whether there's anything in it that couldn't have originated from me alone.
By "God" I mean a substance that is infinite, eternal, unchangeable, independent, all-knowing, and all-powerful — a being by whom I myself and everything else that exists (if anything else does exist) was created. These properties are so extraordinary that the more carefully I consider them, the less it seems possible that this idea could have come from me alone. And so, from everything I've argued above, the conclusion is absolutely necessary: God exists.
Here's why. Yes, I have the idea of substance because I myself am a substance. But I'm a finite being. So I shouldn't have the idea of an infinite substance unless that idea was given to me by a substance that really is infinite.
And I shouldn't suppose that I grasp the infinite merely by negating the finite — the way I understand rest as the absence of motion, or darkness as the absence of light. On the contrary, I clearly see that there is more reality in an infinite substance than in a finite one. In a real sense, my awareness of the infinite comes before my awareness of the finite — my perception of God comes before my perception of myself. How could I know that I doubt, that I desire, that something is lacking in me, that I'm not entirely perfect, unless I already had an idea of a being more perfect than myself, by comparison with which I could recognize my own deficiencies?
And it can't be said that this idea of God is perhaps materially false — that it might have arisen from nothing, from some imperfection in my nature — the way I said earlier about ideas of heat and cold. On the contrary, this idea is supremely clear and distinct, and it contains more representational content than any other idea I have. No idea could be more true or less open to the suspicion of falsity.
This idea of a supremely perfect and infinite being is true in the highest degree. You might imagine that such a being doesn't actually exist, but you can't suppose that the idea of such a being represents nothing real. It is also clear and distinct in the highest degree, because whatever the mind clearly and distinctly conceives as real, true, and involving some perfection is entirely contained in this idea.
It doesn't matter that I can't fully comprehend the infinite, or that there might be an infinity of things in God that I can't comprehend or even conceive of. It's the very nature of the infinite that it can't be fully grasped by a finite being like me. It's enough that I understand this much and judge that everything I clearly perceive, everything I know to involve some perfection — and perhaps also infinitely many properties I'm unaware of — all exist in God, either directly or in some higher form. This is enough to make my idea of God the truest, clearest, and most distinct idea in my mind.
But maybe I'm more than I think I am. Maybe all the perfections I attribute to God somehow exist in me potentially, even if they haven't shown themselves yet. I'm already aware that my knowledge is growing and improving little by little, and I don't see anything to prevent it from increasing indefinitely. And if my knowledge can keep growing, why couldn't I eventually acquire all the other perfections of the divine nature? And if the potential for these perfections already exists in me, why wouldn't that be enough to produce the idea of God?
None of this works, though. Here's why. First, even if it's true that my knowledge is gradually increasing and that there's much in me that hasn't been fully realized yet, none of this comes anywhere close to the idea I have of God, in whom no perfection is merely potential — everything is fully actual. In fact, the very fact that my knowledge grows by degrees is itself a sign of imperfection. Second, even if my knowledge keeps growing, it will never actually become infinite, because it can never reach a point beyond which it's incapable of further growth. But I conceive of God as actually infinite — as a being to whom nothing can be added. Third, the representational content of an idea can't be produced by something that merely has the potential to exist — which, strictly speaking, is nothing — but only by something that actually and formally exists.
To be honest, there's nothing in all of this that isn't easy to see for anyone who thinks about it carefully, by the natural light of reason. But when I let my attention slip even a little, my mind gets clouded by mental images of physical things, and I don't easily remember why the idea of a being more perfect than myself must have come from a being that really is more perfect than me. So let me push the inquiry further: could I, who have this idea of God, exist if there were no God?
Where, in that case, would I get my existence? Perhaps from myself. Or from my parents. Or from some other cause less perfect than God. (Anything more perfect than God, or even equal to God, is inconceivable.)
But suppose I were the source of my own existence. If that were true, I would have no doubts, no unfulfilled desires, no missing perfections. I would have given myself every perfection I can conceive of, and I'd be God. Don't think that the perfections I'm currently missing would be harder to acquire than what I already have. On the contrary, it was clearly a far more difficult thing for me — a thinking being — to emerge from nothing than it would be for me to acquire knowledge of the many things I'm still ignorant of, which are just features of a thinking substance. If I had managed the greater feat of giving myself existence, I certainly wouldn't have denied myself the lesser accomplishments. I wouldn't have denied myself any of the properties I can see are contained in the idea of God, because none of them seem harder to achieve than existence itself. And if any of them were harder, they would certainly seem so to me (assuming I made everything else I have), because they would be the point at which my power ran out.
And even if I supposed that I've always existed just as I am now, that wouldn't get me off the hook. It wouldn't follow that no author of my existence is needed. The whole span of my life can be divided into countless separate moments, each completely independent of the others. The fact that I existed a moment ago doesn't mean I must exist right now, unless some cause creates me afresh at this very instant — that is, sustains me in existence. When you think carefully about the nature of time, it becomes perfectly clear that a thing requires exactly the same power and action to be sustained in existence at each moment as would be needed to create it from scratch if it didn't already exist. The difference between creation and preservation is just a conceptual distinction, not a real one. This is something the natural light makes evident.
So I need to ask myself: do I have some power that can ensure I'll still exist a moment from now? Since I'm a thinking thing (or at least, since the question right now concerns only the thinking part of me), if such a power were in me, I would surely be aware of it. But I'm aware of no such power. This tells me clearly that I depend on some being other than myself.
But maybe that being isn't God. Maybe I was produced by my parents, or by some cause less perfect than God. No — that can't be right. As I argued before, there must be at least as much reality in a cause as in its effect. Since I'm a thinking thing that has an idea of God in me, my cause — whatever it ultimately turns out to be — must also be a thinking thing that possesses the idea of God and all the perfections I attribute to God.
Then the same question can be asked about that cause. Does it get its existence from itself, or from something else? If from itself, then by the reasoning I've already laid out, it must be God. Because if it has the power of self-existence, it must also have the power to actually possess every perfection it can conceive of — in other words, all the perfections I conceive as belonging to God. But if it gets its existence from some other cause, we ask the same question again about that cause, and so on, until we finally arrive at an ultimate cause, which will be God.
And there clearly can't be an infinite chain of causes here. The relevant question isn't what cause originally produced me in the distant past, but what cause is sustaining me in existence right now, at this very moment.
Nor can we suppose that several partial causes worked together to produce me, each contributing one of the perfections I attribute to God, so that all those perfections exist somewhere in the universe but not together in a single being. On the contrary, the unity, simplicity, and inseparability of all God's attributes is itself one of the chief perfections I conceive God to possess. The idea of this unity couldn't have been placed in my mind by any cause that didn't also give me the ideas of all those other perfections. No power could enable me to grasp them as bound together in an inseparable unity without simultaneously giving me knowledge of what they are.
Finally, as for my parents: even granting everything I've ever believed about them, it still doesn't follow that they sustain my existence, or that they produced me insofar as I am a thinking being. At most, they contributed certain physical arrangements to the matter in which I — or rather my mind, which is what I now take myself to be — happens to reside. So my parents create no difficulty here. The conclusion follows absolutely: from the mere fact that I exist and have in me an idea of a supremely perfect being — that is, God — his existence is demonstrated with the greatest clarity.
One question remains: how did I receive this idea of God? I didn't get it from the senses; it doesn't appear to me unexpectedly the way ideas of physical things do when they strike my sense organs. It's also not something I invented or constructed on my own, because I can't add to it or take away from it. The only remaining possibility is that it's innate — placed in me from the very beginning, just as the idea of myself is innate.
And really, it's not surprising that God, in creating me, implanted this idea in me, so that it might serve as the mark of the craftsman stamped on his work. That mark doesn't even need to be something distinct from the work itself. Simply from the fact that God created me, it's highly likely that he fashioned me in some way after his own image and likeness — and that I perceive this likeness through the same faculty by which I'm aware of myself. When I turn my attention inward, I not only find that I am an incomplete, imperfect, and dependent being who constantly strives toward something better and greater than what I am. I also find assurance that the being on whom I depend possesses all the perfections I aspire to — and not just indefinitely or potentially, but actually and infinitely. And that being is God.
The entire force of this argument comes down to this: I couldn't possibly exist with the nature I have — and yet have this idea of God in my mind — unless God really exists. This same God whose idea I find in myself — a being who possesses all those supreme perfections that my mind can dimly conceive but never fully comprehend, a being entirely free from every defect — cannot be a deceiver. Fraud and deception always spring from some deficiency, and the natural light makes clear that a perfect being has no deficiency at all.
But before I examine this further and explore the other truths that follow from it, I want to pause here for a moment to contemplate God himself. I want to reflect at leisure on his astonishing attributes — to behold, admire, and adore this incomparably great light, as far as my mind's eye can bear it, dazzled as it is by the sight. Just as we learn through faith that the supreme happiness of the next life consists in contemplating God, so even now we learn from experience that a similar meditation, though incomparably less perfect, is the source of the deepest satisfaction we can know in this life.
Over the past few days, I've gotten used to pulling my mind away from the senses. I've carefully observed that we actually know very little with certainty about physical objects — that we know much more about the human mind, and still more about God himself. So now I can easily turn my attention away from things I can sense or imagine, and focus instead on things that are purely intelligible — things that exist apart from all matter.
And the idea I have of the human mind — as a thinking thing, not extended in length, width, or depth, sharing none of the properties of a body — is incomparably clearer and more distinct than my idea of any physical object. When I consider that I doubt (in other words, that I'm an incomplete and dependent being), the idea of a complete and independent being — that is, God — comes to mind with such clarity and distinctness that, simply from the fact that this idea exists in me, or that I who possess it exist, I can conclude with total confidence that God exists and that my own existence depends absolutely on him at every moment.
And now I think I can see a path leading from the contemplation of the true God — in whom all the treasures of knowledge and wisdom are contained — to knowledge of everything else in the universe.
Here's why. First, I realize that it's impossible for God ever to deceive me. All fraud and deception involve a certain kind of imperfection. And while the ability to deceive might seem like a sign of cleverness or power, the desire to deceive is clearly evidence of malice or weakness. Neither of those could belong to God.
Second, I'm aware that I have a certain faculty of judgment — the ability to distinguish truth from error — which I undoubtedly received from God, along with everything else I have. Since God can't possibly want to deceive me, I can be sure he hasn't given me a faculty that will lead me into error, as long as I use it correctly.
If that were the whole story, there'd be no problem at all. Because if everything I have comes from God, and God hasn't given me any deceptive faculty, then it seems like I should never make mistakes.
And it's true that when I think only about God — when I see myself as coming from God and turn my attention entirely to him — I find no source of error or falsehood in myself. But the moment I turn back to myself and my own experience, I know perfectly well that I'm subject to countless errors. So where do these errors come from?
When I look for their cause, I notice that my mind contains not only a real, positive idea of God — a supremely perfect being — but also, so to speak, a kind of negative idea of nothingness, of that which is infinitely far from every kind of perfection. I'm something like a middle point between God and nothingness, positioned between absolute existence and nonexistence. To the extent that a supremely perfect being created me, there's nothing in me that could lead me to error. But to the extent that I also participate in nothingness — to the extent that I'm not the supreme being myself, and that I lack many perfections — it's not surprising that I make mistakes.
So I can see that error, considered in itself, isn't some real thing that depends on God. It's simply a deficiency, a lack. To fall into error, God doesn't need to have given me some special faculty for making mistakes. Rather, my errors arise because the power God gave me to tell truth from falsehood isn't infinite.
But this answer isn't entirely satisfying yet. Error isn't merely a lack — not just the absence of some knowledge I'm not supposed to have. It's the absence of knowledge I seemingly ought to have. Think about it this way: if it's true that the more skilled a maker is, the more perfect his products will be, how could something created by the supreme Creator of the universe fail to be absolutely perfect in every way? God certainly could have made me so that I never made mistakes. And God always does what's best. So is it actually better that I'm capable of error than that I'm not?
When I think about this more carefully, several things come to mind.
First, I shouldn't be surprised that I can't always understand why God does what he does. I shouldn't doubt God's existence just because I might encounter things whose reasons I can't grasp. I already know that my nature is extremely weak and limited, while God's nature is immense, incomprehensible, and infinite. So I have no trouble recognizing that God can do an infinity of things whose causes are beyond my understanding. This consideration alone is enough to convince me that the whole category of "final causes" — asking "what was this made for?" — is useless in physics and natural science. It would be reckless of me to think I could figure out God's purposes.
Second, whenever I'm considering whether God's works are perfect, I shouldn't look at any single creature in isolation. I should consider all of God's creatures together, as a whole. Something that might seem highly imperfect if it existed alone in the world might actually be the most perfect thing possible when considered as part of the entire universe. So far in these meditations, I've only established my own existence and God's with certainty. But given what I now know about God's infinite power, I can't deny that God may have created many other things — or at least that he could create them — placing me as just one part of the great whole of creation.
Now, looking more closely at myself, I want to understand what my errors actually are (since errors are the only evidence of imperfection in me). I notice that they depend on two things working together: my faculty of understanding (the intellect) and my faculty of choice (the will).
Through the understanding alone, I neither affirm nor deny anything. I simply grasp ideas about which I might form a judgment. And properly speaking, no error is ever found in the understanding by itself. There may be countless things in the world that I have no idea of, but I can't really say I've been deprived of those ideas — as though I was owed them. There's simply no reason to think God was obligated to give me a bigger intellect than the one I have. However skilled I imagine God to be as a craftsman, I have no grounds for thinking he had to give every one of his creations all the perfections he could give to some of them.
As for my will — I can't complain about that either. In fact, I'm conscious of having a will so vast and far-reaching that it seems to have no limits at all. What strikes me as truly remarkable here is that, of all the faculties I possess, the will is the only one I can't imagine being greater or more perfect. If I consider my understanding, I see immediately that it's quite small and limited. And I can easily form the idea of an understanding that's much greater — even infinitely so. The mere fact that I can conceive of such a thing tells me that an infinite understanding belongs to God's nature. The same goes for my memory, my imagination, and every other faculty I have: I can see that each one is small and limited in me, but immense and infinite in God.
The will, and the will alone, is so great in me that I can't conceive of anything greater. It's mainly this that makes me recognize I bear a certain image and likeness of God. Of course, God's will is incomparably greater than mine — both because of the knowledge and power that accompany it and make it more effective, and because it extends to far more things. But considered in itself, formally and precisely, God's will doesn't seem greater than mine. Here's why: the power of the will consists simply in the fact that we can either do or not do something — affirm or deny, pursue or avoid. Or, more precisely, it consists in the fact that when the understanding presents something to us for judgment, we act in a way that doesn't feel forced by any external power.
Being free doesn't require being equally indifferent between two options. On the contrary, the more I'm drawn toward one option — whether because I clearly see the truth and goodness in it, or because God inclines my thinking that way — the more freely I choose it. Divine grace and natural knowledge, far from reducing freedom, actually increase and strengthen it.
The indifference I feel when nothing pushes me one way rather than another — when I lack a reason to choose — is actually the lowest form of freedom. It reveals a deficiency in knowledge, not a perfection of will. If I always clearly knew what was true and good, I'd never struggle over what to believe or what to choose. I'd be perfectly free without ever being indifferent.
From all this, I can see that neither my will nor my understanding is the source of my errors on its own. My will is enormously wide-ranging and perfect in its kind. My understanding, for its part, correctly grasps whatever it does grasp — since it was given to me by God, everything I actually understand, I understand correctly. Deception in that is impossible.
So where do my errors come from? Here's the answer: they arise from the fact that my will extends further than my understanding. Instead of confining my will to things I clearly understand, I let it range over things I don't understand. When that happens, the will is indifferent — it has no clear perception to guide it — and it easily goes wrong, choosing what's false instead of what's true, or what's bad instead of what's good.
Let me illustrate with an example. When I was recently considering whether anything in the world actually exists, the very act of raising that question made it obvious that I myself must exist. I couldn't help but judge that what I perceived so clearly was true — not because anything forced me to that judgment, but because the great clarity in my understanding naturally produced a strong inclination in my will. The less indifferent I was, the more freely and spontaneously I believed it.
But now I know not only that I exist as a thinking being, but I also have some idea of physical nature. And I find myself in doubt about whether the thinking nature that is in me — or rather, that I am — is different from physical nature, or whether the two are really the same thing. Let's say I don't yet know of any reason that should push me toward one answer over the other. In that case, I'm perfectly indifferent between the two options. It doesn't matter to me which one I affirm or deny — or even whether I form any judgment at all.
And this indifference extends beyond things I know nothing about. It applies generally to everything I don't perceive with perfect clarity at the moment I'm making a judgment. No matter how probable my reasons for leaning one way might be, the simple awareness that they're only probable and not certain is enough to push me toward the opposite conclusion. I had powerful experience of this recently, when I set aside as false everything I'd previously believed to be true, simply on the grounds that I could find some reason to doubt it.
Now here's the key insight. If I hold back from making a judgment when I don't have a clear and distinct perception, I'm acting correctly, and I avoid error. But if I go ahead and affirm or deny anyway, I'm not using my free will properly. If I affirm something false, I'm obviously deceived. And even if I happen to affirm something true, I've only stumbled onto it by luck — I'm still misusing my freedom, because the natural light tells us that the understanding should always guide the will's decisions. It's in this misuse of free will that we find the essential nature of error. Error, as a deficiency, exists in the act insofar as it comes from me. But it doesn't exist in the faculty God gave me, nor in the act insofar as it depends on him.
I certainly have no reason to complain that God didn't give me a more powerful intellect or a brighter natural light. It's in the nature of a finite mind not to understand everything, and it's in the nature of a created mind to be finite. On the contrary, I have every reason to be grateful to God, who owed me nothing, for all the gifts he did give me. I'd be wrong to think he unjustly withheld some perfection that he didn't bestow on me.
I also have no reason to complain that God gave me a will that's broader than my understanding. The will is a single, indivisible thing — you can't take part of it away without destroying it entirely. And really, the more expansive it is, the more grateful I should be to the one who gave it to me.
Finally, I shouldn't complain that God cooperates in producing the acts of my will, including the judgments where I go wrong. Those acts, insofar as they depend on God, are entirely true and good. Being able to perform them is a greater perfection than not being able to. As for the deficiency that constitutes the essence of error — it doesn't require God's involvement, because it's not a real thing at all. When we trace it back to God as its cause, we should call it a "negation" rather than a "privation" (to use the technical language of the philosophers). It's no imperfection in God that he gave me the freedom to assent to or withhold judgment on things he didn't give me clear and distinct knowledge of. But it's clearly an imperfection in me that I don't use this freedom well — that I rush to judge matters I only dimly and confusedly understand.
Still, I can see that God could easily have made me so that I never went wrong, even while leaving me free and limited in knowledge. He could have done this in two ways: either by implanting in my understanding a clear and distinct grasp of everything I'd ever need to make a judgment about, or simply by engraving so deeply on my memory the resolution never to judge anything without first understanding it clearly and distinctly that I'd never forget it. And I understand that, considered as a single being in isolation, I would have been more perfect if God had made me immune to error. But I can't deny that the universe as a whole might actually be more perfect because some of its parts are subject to imperfection than it would be if all parts were exactly alike. And I have no right to complain that the role God assigned me in the world isn't the most distinguished and most perfect of all possible roles.
What's more, even if God didn't give me the first remedy I just described — perfect knowledge of everything — he did leave me the second one: the ability to firmly resolve never to judge anything unless I clearly know it to be true. And although I'm aware of the weakness of not being able to keep my mind focused on one thought indefinitely, I can, through careful and repeated practice, impress this resolution so deeply in my memory that I never fail to recall it when I need it. In this way, I can develop the habit of avoiding error.
And since the highest and most important human perfection consists in being free from error, I believe I've gained something enormously valuable from today's meditation: I've discovered the source of error and falsehood. The source can only be what I've now explained. Whenever I restrain my will so that it makes judgments only about what my understanding clearly and distinctly presents to it, I simply cannot be mistaken. Every clear and distinct perception is something real, and as such it can't come from nothing. It must have God as its source — God, who is supremely perfect and who therefore cannot, without contradiction, be the cause of any error. So every clear and distinct perception must be true.
Today I haven't merely learned what I need to avoid in order to escape error. I've also learned what I need to do to arrive at truth: fix my attention on everything I understand perfectly, and carefully separate those things from what I understand only dimly and confusedly. That is what I intend to do from now on.
There are still several questions left for me to examine about the nature of God and about my own mind. I may return to those on another occasion. For now, what matters most is this: I've discovered what I need to do — and what I need to avoid — in order to arrive at the truth. My chief task now is to work my way out of the doubts I've been stuck in for some time, and to figure out whether anything can be known with certainty about material objects — things in the physical world.
But before I ask whether such objects actually exist outside my mind, I need to examine my ideas of them, as they exist in my own consciousness, and sort out which ideas are clear and distinct and which are confused.
First, I can clearly imagine what philosophers usually call "continuous quantity" — that is, the extension of an object in length, width, and depth. I can mentally divide such a thing into many distinct parts, and I can assign to each part all sorts of sizes, shapes, positions, and motions. And I can assign to each of these motions any duration I like.
I don't only know these things in a general way. When I pay closer attention, I discover countless specific truths about shapes, numbers, motion, and similar topics. These truths are so self-evidently correct, and so natural to my mind, that when I encounter them, it doesn't feel like I'm learning something new at all. It feels more like I'm remembering something I already knew — or noticing, for the first time, something that was already present in my mind but that I'd never focused on before.
The most important thing I notice here is this: I find in my mind countless ideas of things that have their own true and unchangeable natures, even if those things have no reality outside my thought. These ideas aren't just empty negations — nothing at all. And they aren't things I've invented, even though it's up to me whether I think about them or not. They have fixed, immutable essences of their own.
Take the triangle as an example. Maybe there has never been a triangle anywhere in the physical universe, outside my thought. Nevertheless, the triangle has a definite nature — a form or essence — that is unchanging and eternal, that wasn't created by me, and that doesn't depend on my thinking in any way. I know this because various properties of the triangle can be demonstrated: its three angles are equal to two right angles, its longest side is opposite its largest angle, and so on. Whether I want them to or not, I now clearly recognize that these properties belong to the triangle. And yet I never thought about any of these properties when I first imagined a triangle. So they can't be something I invented.
Someone might object: maybe the idea of a triangle came into my mind through the senses — maybe I saw triangular-shaped objects and formed the idea from those. But that doesn't hold up. I can form in my mind an endless variety of shapes that I've certainly never encountered through the senses, and I can still demonstrate various properties of those shapes, just as I can with the triangle. All of these properties are certainly true, because I conceive them clearly. And since they're true, they must be something real — not mere nothingness. (After all, everything that is true is something real, and truth goes hand in hand with reality.) I've already shown at length that whatever I perceive clearly and distinctly is true.
And even if I hadn't proved that principle, my mind is simply built in such a way that I can't help assenting to what I clearly perceive, at least while I'm perceiving it clearly. I can recall that even back when I was still fully attached to the world of the senses, I counted the truths of geometry, arithmetic, and pure mathematics among the most certain things I knew.
—-
Now here is the crucial move. If the mere fact that I can draw an idea out of my thought means that everything I clearly and distinctly perceive as belonging to that idea really does belong to it — then can't I use this same principle to prove that God exists?
Here is how. I find the idea of God in my mind — that is, the idea of a supremely perfect being — just as surely as I find the idea of any shape or number. And I know, with no less clarity and distinctness, that eternal existence belongs to God's nature than I know that the properties I can demonstrate of a shape or number really belong to that shape or number. So even if everything I concluded in the earlier Meditations turned out to be wrong, God's existence should still strike me as at least as certain as any truth of mathematics.
Now, I admit this might seem like sophistry at first glance. In every other case, I'm used to distinguishing between a thing's existence and its essence — between what something is and whether it actually exists. So it's easy to think that existence can be separated from God's essence, and that therefore God can be conceived of as not actually existing.
But when I think more carefully, I see that existence can no more be separated from God's essence than the concept of a valley can be separated from the concept of a mountain, or the fact that its three angles equal two right angles can be separated from the essence of a triangle. It's simply impossible to conceive of God — a supremely perfect being — as lacking existence. That would be like conceiving of a mountain without a valley. A supremely perfect being that doesn't exist would be a being that lacks a perfection, which is a contradiction.
But wait — someone might push back. Even though I can't conceive of a mountain without a valley, it doesn't follow that any mountain or valley actually exists in the world. Just because those two concepts are inseparable in my thinking doesn't mean the things themselves have to exist. So why should it be any different with God? Just because I conceive of God as existing, it doesn't seem to follow that God actually exists. After all, my thinking doesn't impose any necessity on reality. Just as I can imagine a winged horse even though no such thing exists, maybe I'm just attaching existence to God, even though no God exists.
But these two cases are not actually analogous, and there's a subtle error hidden in this objection. Here's the key difference: when I say I can't conceive of a mountain without a valley, all that follows is that mountains and valleys are inseparable concepts — whether or not any mountain or valley actually exists. But when I say I can't conceive of God without existence, something stronger follows: existence is inseparable from God, and therefore God truly exists. This isn't something my thought is imposing on reality. It's the other way around: the necessity that lies in the thing itself — the necessity of God's existence — is what determines me to think this way. I am not free to think of God without existence, because a supremely perfect being that lacks the absolute perfection of existence is simply inconceivable. It's not like imagining a horse with or without wings, where either option is equally easy to think.
Someone might raise another objection. Sure, they might say, once you've assumed that God has all perfections, you have to admit he exists — since existence is one of those perfections. But the initial assumption wasn't necessary. You didn't have to start by thinking that God has all perfections. It's like saying: once I assume that all four-sided figures can be inscribed in a circle, I'd have to admit that a rhombus can be inscribed in a circle, since a rhombus has four sides. But that conclusion is obviously false — and the problem is that the original assumption was wrong. Maybe the same thing is happening with God.
But this objection doesn't work either. It's true that I don't have to think about God at any particular moment. But whenever I do happen to think of a first and supreme being — whenever I pull this idea out from the storehouse of my mind, so to speak — I am compelled to attribute all perfections to him, even if I don't list every single one of them at that moment. This necessity is enough: as soon as I recognize that existence is a perfection, I'm forced to conclude that this first and supreme being exists.
It's exactly like the triangle. I don't have to think about triangles at any given moment. But whenever I do consider a three-sided, straight-lined figure, I must attribute to it the properties from which it follows that its three angles are no greater than two right angles — even if I'm not thinking about that specific property at the time.
The case of the quadrilateral is completely different. When I consider which shapes can be inscribed in a circle, there's no necessity at all to believe that all four-sided figures can be. In fact, I can't even imagine this to be the case, as long as I'm committed to accepting only what I clearly and distinctly conceive. So there is a vast difference between false assumptions, like the one about quadrilaterals, and the true ideas that are innate in me — the first and most important of which is the idea of God.
I can tell on many grounds that this idea of God isn't something I made up. It's the representation of a true and immutable nature. First, I can't conceive of any other being whose essence necessarily includes existence. Second, it's impossible to conceive of two or more Gods of this kind. Third, if one such God exists, I can clearly see that he must have existed from all eternity and will continue to exist for all eternity. And finally, I perceive many other properties in God, none of which I can diminish or alter.
—-
But whatever line of proof I ultimately follow, it always comes back to this: the only things that fully persuade me are the ones I conceive clearly and distinctly. And while some of the things I conceive in this way are obvious to everyone, others are only discovered after careful investigation. Yet once discovered, the latter are just as certain as the former. For example: in a right-angled triangle, it isn't as immediately obvious that the square of the hypotenuse equals the sum of the squares of the other two sides as it is that the hypotenuse is opposite the largest angle. But once you've grasped both truths, you're just as certain of one as of the other.
As for God: if my mind weren't weighed down by prejudices and constantly bombarded by images from the senses, there is nothing I would know sooner or more easily than the fact of his existence. For is there any truth more self-evident than that a Supreme Being exists — a being to whose essence alone necessary and eternal existence belongs?
Although it took me quite a bit of careful thought to reach a firm understanding of this truth, I now feel not only as certain of it as of anything else I hold to be most certain, but I also notice something further: the certainty of every other truth depends entirely on this one. Without the knowledge that God exists, it would be impossible ever to know anything with perfect certainty.
Here is why. My nature is such that, while I'm in the process of clearly and distinctly perceiving something, I can't help but believe it's true. But my nature is also such that I can't keep my mind focused on the same thing forever. I often recall a past judgment without being able to recall the reasoning that led me to it. And if I didn't know that God existed, other considerations could come along in the meantime and easily change my mind. Without God, I would have no true and certain knowledge — only shifting, unreliable opinions.
For example, when I contemplate the nature of a triangle, it seems absolutely clear to me — having studied geometry — that its three angles equal two right angles. I find it impossible to believe otherwise while I'm focused on the proof. But as soon as I stop attending to the proof, even though I remember that I understood it clearly, I could easily begin to doubt the conclusion — if I didn't know that God exists. I might convince myself that nature made me the kind of being who gets deceived even about things that seem most evident and certain. After all, I've often thought things were true and certain, only to discover later that they were completely false.
But once I've discovered that God exists, and have also recognized that everything depends on him and that he is no deceiver, then I can draw this conclusion: everything I perceive clearly and distinctly must necessarily be true. Even when I'm no longer paying attention to the reasons that led me to a judgment, no opposing argument can be strong enough to make me doubt it — as long as I remember that I once understood it clearly and distinctly. At that point, my knowledge becomes true and certain.
And this certainty extends to everything else I remember having demonstrated, including the truths of geometry and similar fields. What objection could anyone raise against them? That my nature might be such that I'm frequently deceived? But I now know that I can't be deceived about things whose grounds I clearly understand. That I once believed things to be true that I later found to be false? But none of those things were clearly and distinctly known to me. Back then, I didn't yet have the rule that tells me what makes a judgment certain, so I gave my assent based on reasons that I later realized were weaker than I'd thought. What else could be objected? Maybe that I'm dreaming? I raised that worry myself earlier. But even if I'm dreaming, the principle still holds: whatever is presented clearly to my intellect is absolutely true.
And so I see, with perfect clarity, that the certainty and truth of all knowledge depends entirely on the knowledge of the true God. Before I knew him, I could not have perfect knowledge of anything. Now that I do know him, I have the means to achieve perfect knowledge of countless things — both about God himself and about other things grasped by the intellect, as well as about physical nature, insofar as it is the subject of pure mathematics.
The only question left to investigate is whether physical things exist. On this question, I at least know for certain that they can exist, insofar as they are the subject matter of pure mathematics, since I can conceive of them clearly and distinctly in that respect. There's no doubt that God has the power to produce everything I can distinctly conceive, and I've never considered anything impossible for him unless I ran into a contradiction when trying to conceive it properly. Furthermore, the faculty of imagination that I possess — a faculty I'm aware of using whenever I turn my attention to physical things — gives me reason to think they exist. Here's why: when I carefully consider what imagination actually is, I find that it's simply a particular application of my knowing faculty to a body that is immediately present to it, and which therefore exists.
To make this clearer, let me point out the difference between imagination and pure understanding. Take this example: when I imagine a triangle, I don't merely understand that it's a figure with three sides. At the same time, I also visualize those three sides as present before my mind's eye, through a kind of inner mental focus. That's what I call imagining. But if I want to think about a chiliogon — a thousand-sided figure — I can certainly understand that it's a figure with a thousand sides, just as easily as I understand that a triangle has three. But I can't imagine those thousand sides the way I can the three sides of a triangle. I can't picture them as present before my mind's eye. And although, out of habit, I always try to picture something whenever I think about physical objects, so that in thinking of a chiliogon I may vaguely picture some sort of figure in my mind, this picture is clearly not a chiliogon. It's no different from the image I'd form if I were thinking of a myriogon — a ten-thousand-sided figure — or any other figure with a lot of sides. This vague picture would be useless for discovering the properties that distinguish a chiliogon from other polygons. But if the question is about a pentagon, it's true that I can understand its shape without the help of imagination, just as I can with a chiliogon. But I can also imagine it, by focusing my mind's attention on its five sides and the area they enclose. So I notice that a special mental effort is needed for the act of imagining — an effort not required for pure understanding. This special effort clearly reveals the difference between imagination and pure intellection.
I'd also note that this power of imagination, insofar as it differs from the power of pure understanding, is in no way essential to who I am — to the essence of my mind. Even if I didn't have it, I would still be the same person I am now. From this, it seems we can conclude that imagination depends on something other than the mind. And I can easily see how it would work: if some body exists, and my mind is so closely connected and united with it that it can turn to examine it at will, then this would explain how the mind imagines physical objects. The difference between imagining and pure understanding would then come down to this: when the mind purely understands, it turns inward and considers one of the ideas it already contains within itself. But when it imagines, it turns outward toward the body and perceives in it something that corresponds to an idea it has formed on its own or received through the senses. I can easily see, I say, that imagination could work this way — if it's true that bodies exist. And because I can't find any other obvious way to explain it, I conjecture with some probability that they do exist. But only with probability. Even after careful examination, I don't find that I can draw a necessary proof of the existence of any body from the distinct idea of physical nature that I have in my imagination.
Now, I'm used to imagining many things beyond the physical nature that pure mathematics studies — things like colors, sounds, tastes, pain, and so on — though with less distinctness. And since I perceive these things much better through the senses, which seem to have delivered them to my imagination via memory, it seems right that I should also examine what sense perception is and see whether I can get a definitive proof of the existence of physical objects from those ideas that come to me through this mode of awareness.
First, I'll recall what I previously accepted as true on the basis of sense perception, and what my grounds were for believing it. Second, I'll review the reasons that later forced me to doubt it. And finally, I'll consider what I should believe now.
So, to begin: I perceived that I had a head, hands, feet, and other body parts composing what I considered to be part of myself — or perhaps even all of myself. I also perceived that this body was situated among many others that could affect it in various ways, both good and bad. I noticed what was good through a certain sensation of pleasure, and what was bad through a sensation of pain. Besides pleasure and pain, I also experienced hunger, thirst, and other appetites, along with physical inclinations toward joy, sadness, anger, and similar emotions. And looking beyond myself, I perceived in bodies not only their extension, shape, and movement, but also hardness, heat, and other qualities of touch, as well as light, colors, smells, tastes, and sounds. The variety of these let me distinguish the sky from the earth, the sea from everything else, and one body from another.
Given the ideas of all these qualities that were presented to my mind — which were, strictly speaking, the only things I directly perceived — it wasn't unreasonable for me to think I was perceiving objects entirely different from my thoughts, namely, bodies that produced those ideas. I was aware that the ideas came to me without my choosing them: I couldn't perceive any object, no matter how much I wanted to, unless it was present to one of my sense organs, and I couldn't avoid perceiving it when it was present. Because the ideas I received through the senses were far more vivid, clear, and in their own way more distinct than anything I could produce by deliberate thought or find stored in memory, they seemed unable to have come from me alone and must therefore have been caused by other objects. And since the only knowledge I had of those objects came from the ideas themselves, the most natural conclusion was that the objects resembled the ideas they caused. I also remembered that I had relied on the senses before I relied on reason, and that the ideas I formed on my own were never as clear as those I received through the senses — and were mostly assembled from pieces of the latter. So I easily convinced myself that there was no idea in my understanding that hadn't first passed through the senses.
And I wasn't entirely wrong to believe that the body I called my own, by a special right, belonged to me more closely than any other body. I could never be separated from it the way I could from other bodies. I felt all my appetites and emotions in it and because of it. Pain and the stirring of pleasure affected its parts, not the parts of other bodies separate from it. But when I asked why a certain mysterious sensation of pain should give rise to sadness, or why pleasure should produce joy, or why that indescribable twisting in the stomach I call hunger should make me think of eating, and dryness in the throat should make me think of drinking, and so on — I couldn't give any explanation, except that nature taught me so. There's certainly no obvious connection — at least none I can understand — between that irritation in the stomach and the desire to eat, any more than between perceiving something that causes pain and the feeling of sadness that follows. All my other judgments about the objects of sense seemed to be dictates of nature in the same way, because I noticed that these judgments formed in me before I had any chance to weigh the reasons for or against them.
But over time, experience gradually eroded the trust I had placed in my senses. Towers that looked round from a distance turned out to be square up close. Colossal statues on top of those towers looked tiny when viewed from the ground. In countless cases like these, I found that judgments based on the external senses were wrong. And not just the external senses — even the internal ones. What could be more internal than pain? Yet I learned from people who'd had an arm or leg amputated that they still sometimes seemed to feel pain in the missing limb. This made me think I couldn't be entirely sure that any of my own limbs was actually affected when I felt pain in it.
To these grounds for doubt, I soon added two more of very broad scope. The first was this: I believed that nothing I ever perceived while awake couldn't also seem to occur in my dreams. And since I didn't think the ideas in my dreams came from objects outside me, I couldn't see any reason to believe this about what I seemed to perceive while awake either. The second was this: since I didn't yet know who made me (or at least was supposing I didn't), there was nothing to prevent me from having been made in such a way that I was deceived even about what seemed most obviously true. As for the reasons that had previously convinced me of the existence of physical objects, I had no great difficulty answering them. Since nature seemed to push me toward many things that reason warned me against, I didn't think I should trust its teachings very much. And although my sense perceptions didn't depend on my will, I didn't think that proved they came from things outside me. Perhaps some faculty within me — one I hadn't yet discovered — was producing them.
But now that I'm coming to know myself better and to see more clearly who made me, I don't think I should rashly accept everything the senses seem to teach. On the other hand, I don't think I should doubt all of it either.
First of all: since I know that everything I clearly and distinctly conceive can be made by God exactly as I conceive it, the mere fact that I can clearly and distinctly conceive one thing apart from another is enough to be certain they are different — since at least God could make them exist separately. It doesn't matter what power actually separates them; the point is that I'm able to conceive them apart. So, simply because I know with certainty that I exist, and because in the meantime I can't see that anything else necessarily belongs to my nature beyond my being a thinking thing, I rightly conclude that my essence consists solely in being a thinking thing — a substance whose entire essence or nature is simply to think. And even though I may possess a body (indeed, as I'll shortly explain, I certainly do possess a body) with which I am very closely joined, nevertheless — because on the one hand I have a clear and distinct idea of myself as simply a thinking, non-extended thing, and on the other hand I have a distinct idea of body as simply an extended, non-thinking thing — it is certain that I (that is, my mind, by which I am what I am) am entirely and truly distinct from my body, and can exist without it.
Furthermore, I find within myself various faculties of thought, each with its own particular character. For example, I have the faculties of imagining and perceiving. I can clearly and distinctly conceive of myself as a complete being without these faculties, but I can't conceive of them without me — that is, without an intelligent substance in which they exist. In their very nature, they involve some form of understanding, which tells me they are distinct from me in the way that modes are distinct from the things they belong to. I also notice certain other faculties — like the power to change location, to take on various shapes, and so on — that likewise can't be conceived of or exist apart from some substance. But it's clear that these faculties, if they truly exist, must belong to some physical or extended substance, since their very concept involves extension but involves no understanding at all.
Beyond that, there's a certain passive faculty of perception in me — a capacity for receiving and becoming aware of the ideas of sensible things. But this faculty would be useless unless there also existed, either in me or in something else, an active faculty capable of forming and producing those ideas. Now, this active faculty can't be in me, at least not insofar as I'm merely a thinking thing, since it doesn't presuppose any thinking on my part and since these ideas are frequently produced in my mind without any contribution from me — and often contrary to my will. So this active faculty must exist in some substance other than me, and all the representational content of the ideas it produces must be contained in that substance — either formally (in the same form) or eminently (in a higher form), as I established earlier. This substance is either a body — a physical thing that formally contains everything that is representationally present in my ideas — or it is God himself, or some being more exalted than body, which contains it eminently.
But since God is no deceiver, it's clear that he doesn't send me these ideas directly from himself, nor does he use some intermediary creature that contains their representational content only eminently rather than formally. He has given me no ability to discover that this is the case. On the contrary, he has given me a very strong inclination to believe that these ideas come from physical objects. I don't see how God could be defended against the charge of deception if these ideas actually came from any source other than physical things. Therefore, physical objects must exist.
That said, they may not be exactly the way we perceive them through the senses, since sense perception is often quite obscure and confused. But at least we must acknowledge that everything I clearly and distinctly conceive as belonging to them — that is, broadly speaking, everything that falls within the scope of pure geometry — really does exist outside of me.
As for other properties that are either merely particular (for example, that the sun is of such-and-such a size and shape) or are conceived with less clarity and distinctness (like light, sound, pain, and so on) — although these are quite uncertain, nevertheless, on the sole ground that God is no deceiver and therefore hasn't allowed any falsehood in my beliefs without also giving me the means to correct it, I think I can safely conclude that I have the resources within myself to arrive at the truth about them. And first of all, there can be no doubt that each of nature's teachings contains some truth. By "nature" in its most general sense, I now mean nothing other than God himself, or the order and arrangement God has established in created things. And by my nature in particular, I mean everything that God has given to me.
Now, there is nothing nature teaches me more vividly than that I have a body that feels bad when I'm in pain and that needs food and drink when I feel hungry or thirsty. I should therefore have no doubt that there's some truth in this.
Nature also teaches me, through these sensations of pain, hunger, thirst, and so on, that I'm not merely lodged in my body the way a pilot is stationed in a ship. Rather, I am so closely joined to it, so thoroughly intermingled with it, that my mind and body together form a kind of unity. If this weren't the case, then when my body was hurt, I — being simply a thinking thing — wouldn't feel pain. Instead, I would perceive the injury purely through my understanding, the way a pilot sees by sight when something in the ship is damaged. And when my body needed food or drink, I would have clear intellectual knowledge of this fact, rather than being alerted to it by the confused sensations of hunger and thirst. For in truth, all these sensations of hunger, thirst, pain, and the rest are really nothing more than certain confused modes of thinking that arise from the union and apparent merging of mind and body.
Beyond this, nature teaches me that my body is surrounded by many other bodies, some of which I should seek out and others I should avoid. And indeed, since I perceive various kinds of colors, sounds, smells, tastes, heat, hardness, and so on, I can safely conclude that the bodies from which these different sense perceptions come must have certain features that correspond to those perceptions — though these features may not actually resemble them. And since some of these perceptions are pleasant and others unpleasant, there can be no doubt that my body — or rather my complete self, insofar as I'm made up of both body and mind — can be affected in various ways, both good and bad, by the bodies around it.
But there are many other things that seem to be teachings of nature but really aren't. They're just beliefs that crept into my mind through a habit of forming careless judgments. It can easily happen that such judgments contain errors. For example, I've believed that all space in which there's nothing affecting my senses must be empty. That whatever is hot must contain something exactly like the idea of heat in my mind. That a white or green body must actually contain the same whiteness or greenness that I perceive. That whatever tastes bitter or sweet must have that very taste in it. That the stars, towers, and all distant objects must be the same size and shape as they appear to my eyes. And so on.
But to avoid any confusion here, I need to define more precisely what I mean by "being taught by nature." I'm using "nature" here in a narrower sense than when it means the totality of everything God has given me. In that broader sense, "nature" would include much that belongs only to the mind — things I'm not referring to at the moment — such as my grasp of the truth that what is done cannot be undone, and all other truths I perceive by the natural light of reason, without any help from the body. It would also include much that belongs only to the body and that I'm not calling "nature" here either, such as the property of heaviness and the like. I'm using "nature" exclusively to refer to what God has given me as a being composed of both mind and body.
So nature, taken in this sense, does teach me to avoid what causes pain and to pursue what gives pleasure, and things of that sort. But I don't find that nature teaches me to draw conclusions about things in the external world based directly on these sense perceptions, without first examining them carefully with my mind. It seems to me that discerning the truth about external objects is the job of the mind alone, not of the composite whole of mind and body.
For example, although a star makes no bigger impression on my eye than a candle flame does, I don't feel any natural or instinctive urge to believe the star is actually no bigger than the flame. I simply judged it that way since childhood, without any rational basis. And even though approaching fire makes me feel heat, and getting too close makes me feel pain, there's no reason to conclude that there's something in the fire that actually resembles my sensation of heat, any more than that there's something resembling my sensation of pain. All I have grounds for believing is that there's something in the fire, whatever it may be, that produces those sensations of heat or pain in me. Similarly, even though there are spaces where nothing excites my senses, I shouldn't conclude that those spaces contain nothing at all. I can see that on this point, as on many others, I've been in the habit of turning nature's order upside down. The perceptions of the senses were given to me by nature merely as signals to tell my mind what is beneficial or harmful to the composite being of which it is a part, and they are clear and distinct enough for that purpose. But I've been using them as infallible rules for determining the true nature of external objects — a task for which they can only provide the most obscure and confused information.
I've already examined thoroughly enough how it is that, despite the supreme goodness of God, my judgments can still contain errors. But a difficulty arises here concerning the things nature teaches me to pursue or avoid, and concerning my internal sensations, in which I seem to have sometimes detected error — cases where nature itself seems to deceive me directly. Take, for example, the case of someone who is tricked by the pleasant taste of food that has been laced with poison. Nature leads this person to desire the food because of its pleasant taste, not the poison, which nature knows nothing about. All we can conclude from this is that our nature isn't all-knowing, which is hardly surprising, since we are finite beings with correspondingly limited knowledge.
But we also quite frequently make errors about things to which nature directly inclines us. Consider people with dropsy, who crave drink even though it would harm them. You might say the reason they're deceived is that their nature has been corrupted. But this doesn't really solve the problem, because a sick person is just as much a creature of God as a healthy one. It seems just as contradictory to the goodness of God that the sick person's nature should be deceptive as that the healthy person's should be.
Now, consider a clock made of wheels and counterweights. It follows the laws of nature no less precisely when it's poorly made and tells the wrong time than when it fulfills its maker's intentions perfectly. In the same way, if I think of the human body as a kind of machine — assembled from bones, nerves, muscles, veins, blood, and skin — arranged so that it would go through the same involuntary motions it currently exhibits even if there were no mind in it, then I can easily see that it would be just as natural for such a body, if it were dropsical, to experience the dryness of the throat that normally triggers the sensation of thirst, and to have its nerves and other parts move in the way required for drinking (thereby making the illness worse and harming itself), as it would be natural for a healthy body to be prompted to drink for its own good by a similar dryness. And although, looking at the purpose a clock was designed for, I might say that a clock that tells the wrong time has strayed from its proper nature; and on the same principle, looking at the human body as a machine designed by God for its usual functions, I might say that a dropsical body whose dry throat doesn't lead to beneficial drinking has gone wrong in its nature — still, I can plainly see that this second use of "nature" is very different from the first. The second is nothing more than a label that depends entirely on my thinking, something external that I apply by comparing a sick person and a badly made clock to my ideas of a healthy person and a well-made clock. But the first use of "nature" refers to something genuinely found in things — something that has real truth.
Now, it's true that when we talk about a dropsical body whose nature is "corrupted" because its throat is dry even though it doesn't need a drink, we're applying an external label. But when we talk about the composite whole — the mind in its union with the body — it's not just a label. It is a genuine error of nature for this composite being to feel thirst when drinking would be harmful. And so we still need to ask why God's goodness doesn't prevent this kind of error in human nature.
To begin this investigation, I'll first note that there is an enormous difference between mind and body. By its very nature, body is always divisible, while mind is completely indivisible. When I consider the mind — that is, when I consider myself purely as a thinking thing — I can't distinguish any parts within myself. I perceive myself as something absolutely unified and whole. And even though the entire mind seems to be united to the entire body, if a foot or an arm or any other part is cut off, I'm aware that nothing has been taken away from my mind. The faculties of willing, perceiving, understanding, and so on can't properly be called its "parts," because it's one and the same mind that wills, that perceives, and that understands.
The exact opposite is true of physical or extended things. I can't think of any physical object, no matter how small, that I can't easily divide in thought — and that I don't therefore recognize as divisible. This alone would be enough to teach me that the mind is entirely different from the body, if I hadn't already established this on other grounds.
Next, I note that the mind doesn't receive impressions from every part of the body directly. It receives them only from the brain — or perhaps from just one small part of it, namely, what is called the "common sense." Whenever this part of the brain is affected in the same way, it produces the same perception in the mind, regardless of how the rest of the body might be arranged. This is proven by countless experiments that don't need to be listed here.
I also note that the nature of the body is such that whenever any one of its parts is moved by another part some distance away, it can just as easily be moved in the same way by any of the parts in between, even if the more distant part does nothing. For example, consider a taut cord A-B-C-D. If you pull the last part, D, the first part, A, will move in exactly the same way it would if you had pulled one of the middle parts, B or C, and left D alone. In the same way, when I feel pain in my foot, physics teaches me that this sensation is produced by means of nerves that stretch from the foot to the brain. When these nerves are disturbed in the foot, they simultaneously disturb the innermost part of the brain where they originate, exciting there a specific motion that nature has established to cause the mind to feel a sensation of pain as if located in the foot. But since these nerves must pass through the shin, the thigh, the lower back, the upper back, and the neck to reach the brain, it can happen that even though the nerve endings in the foot aren't affected — only certain parts along their path through the lower back or neck — the same motion is excited in the brain as would be caused by an actual injury to the foot. The mind will then necessarily feel pain in the foot, exactly as if it had actually been hurt. And the same applies to all our other sense perceptions.
Finally, I note that since each movement in the part of the brain that directly affects the mind produces only one particular sensation, the best arrangement one could imagine is for that movement to cause, out of all the sensations it could possibly produce, the one that is most useful for preserving the health of the human body. And experience shows that this is exactly what all the perceptions nature has given us are like. There is nothing in them that doesn't demonstrate the power and goodness of God.
For example, when the nerves of the foot are shaken violently or unusually hard, the motion travels through the spinal cord to the innermost parts of the brain and gives the mind a signal — a sensation of pain felt as if it were in the foot. This alerts the mind and motivates it to do everything it can to remove the cause of the pain, since it's dangerous and harmful to the foot. Now, it's true that God could have designed human nature so that the same motion in the brain caused the mind to be aware of something entirely different. The motion might have made the mind conscious of itself insofar as it exists in the brain, or insofar as it exists somewhere between the foot and the brain, or it might have caused the mind to perceive some completely different thing. But none of those alternatives would have been as useful for preserving the body as what the mind actually feels. In the same way, when we need something to drink, a certain dryness arises in the throat that moves its nerves, and through them the internal parts of the brain. This movement gives the mind the sensation of thirst, because in that situation nothing is more useful for us than to know that we need drink for the preservation of our health. And so it goes in all the other cases.
From all of this, it is quite clear that despite the supreme goodness of God, human nature — insofar as it is composed of mind and body — can't help but sometimes lead us astray. If some cause produces in the brain the same motion that is normally produced when the foot is injured — but the cause this time is not in the foot but somewhere along the nerves between the foot and the brain, or even in the brain itself — then pain will be felt as if it were in the foot, and the senses will be naturally deceived. Since the same motion in the brain can only produce the same sensation in the mind, and since this sensation is far more often caused by something actually hurting the foot than by something acting elsewhere, it's reasonable that this motion should lead the mind to feel pain in the foot rather than in any other part of the body. And if dryness of the throat sometimes arises not, as usual, because drinking would be good for the body, but from the opposite cause — as happens in dropsy — it's still much better for it to mislead us in that case than for it to mislead us constantly when the body is healthy. The same reasoning applies to all the other cases.
This line of thinking is enormously useful, not only for recognizing the errors my nature is prone to, but also for making it easier to avoid or correct them. I know that all my senses usually point me toward what is true rather than what is false in matters relating to the body's well-being. I'm almost always able to use more than one sense to examine the same object. I can use my memory to connect present experience with past knowledge. And I can use my understanding, which has by now uncovered all the sources of my errors. So I should no longer fear that the things my senses present to me every day are false. The extravagant doubts of those earlier days should be dismissed as overblown and absurd — especially the most sweeping doubt of all, the one about sleep, which I couldn't distinguish from being awake.
For I now find a very clear difference between the two. Our memory can never connect our dreams with one another and with the rest of our lives the way it habitually connects the events that occur when we're awake. Truly, if someone appeared to me suddenly while I was awake and then vanished just as suddenly — the way images do in dreams — so that I couldn't tell where they came from or where they went, I would have every reason to consider them a ghost or a phantom produced by my brain rather than a real person. But when I perceive things and can clearly determine where they come from, where they are, and when they appeared to me, and when I can connect that perception without interruption to the rest of my life, then I am perfectly certain that I'm perceiving them while awake and not in a dream.
And I should not have the slightest doubt about the truth of these perceptions if, after calling on all my senses, my memory, and my understanding to examine them, none of these faculties reports anything that contradicts any other. Since God is no deceiver, it necessarily follows that in such cases I am not being deceived.
But because the demands of practical life often force us to make decisions before we've had time for such careful examination, we must acknowledge that human life is frequently subject to error about particular things. And in the end, we must recognize the weakness of our nature.
Sir — Your translation of my Principles is so elegant and polished that I expect the work will be more widely read in French than in Latin, and better understood too. My only worry is that the title might scare off people who weren't raised on academic texts, or who have a low opinion of philosophy because the kind they were taught turned out to be worthless. That makes me think it would be useful to add a preface explaining what the book is about, what I was trying to accomplish in writing it, and what readers can gain from it. Now, I should probably be the one to write this kind of preface, since I know these things better than anyone else. But I can't bring myself to do more than sketch out the main points I think ought to be covered. I'll leave it to your judgment which parts to share with the public.
First, I would have liked to explain what philosophy is, starting with the most basic ideas. The word "philosophy" means the pursuit of wisdom, and by wisdom I don't just mean street smarts or good judgment in practical affairs. I mean a complete knowledge of everything a human being can know — knowledge that helps us conduct our lives well, preserve our health, and advance all the practical arts and sciences. To achieve this kind of knowledge, we have to trace things back to their most fundamental causes. So to truly study philosophy — to "philosophize" in the proper sense — we have to begin by investigating those fundamental causes, which are called "principles."
These principles must meet two conditions. First, they must be so clear and self-evident that the human mind, when it focuses on them carefully, cannot doubt their truth. Second, all our other knowledge must depend on them in such a way that while the principles can be understood on their own, everything that follows from them cannot be understood without them. From there, we need to work out, step by step, all the knowledge that flows from these principles, making sure that every link in the chain of reasoning is perfectly clear. In the strictest sense, only God is truly wise — only God has perfect knowledge of all things. But we can say that some people are more or less wise depending on how much they know of the most important truths. And I'm confident that every educated person would agree with everything I've said so far.
Next, I would have wanted to discuss the usefulness of philosophy. Since philosophy encompasses everything the human mind can know, we should recognize that it's what sets us apart from people living in ignorance and barbarism. The level of civilization and culture in any nation corresponds directly to how much genuine philosophy flourishes there. Having true philosophers is the greatest advantage a nation can enjoy. Beyond that, for individuals, it's not only useful to spend time with people who study philosophy — it's incomparably better to study it yourself. It's obviously preferable to use your own eyes to find your way and enjoy the beauty of color and light, rather than blindly following someone else's guidance. Though following a guide is certainly better than shutting your eyes and having nobody to lead you at all. Living without philosophizing is really the same as keeping your eyes shut without ever trying to open them. And the pleasure of seeing everything that sight reveals is nothing compared to the satisfaction that philosophical discovery provides. In fact, this study is even more essential for living a good life than eyesight is for walking down the street.
Animals, which have only their bodies to take care of, spend all their time looking for food. But human beings, whose most important part is the mind, ought to make the search for wisdom their primary concern — because wisdom is the true nourishment of the mind. I'm also confident that many people would succeed in this search if only they believed it was possible and understood what they were capable of. There's no mind so limited that it's permanently stuck on what the senses deliver. Everyone, at some point, turns away from sensory things and reaches for something higher — even if they don't know exactly what that higher good is. The luckiest people in the world — those with health, honors, and riches in abundance — aren't any more immune to this longing than anyone else. In fact, I'm convinced that these are the people who sigh most deeply for a good that's greater and more perfect than anything they already have. But the highest good, as far as natural reason can determine without the guidance of religious faith, is nothing other than knowledge of truth through its fundamental causes. In other words, it's the wisdom that philosophy pursues. All of this is clearly true. All that's needed to convince people of it is to state it well.
But experience seems to argue against these claims, because people who call themselves philosophers are often less wise and reasonable than those who never studied the subject at all. So I should briefly explain what all the knowledge we currently possess amounts to, and what levels of wisdom we've actually reached. The first level includes only ideas so obvious that they can be grasped without any special reflection. The second level is everything we learn from the experience of our senses. The third is what we pick up from conversations with other people. And the fourth is reading — not all books, but especially those written by people who actually have something worthwhile to teach, since reading is really a kind of conversation with the authors. It seems to me that all the ordinary wisdom people possess comes from these four sources alone. I'm not counting divine revelation here, because revelation doesn't take us through a gradual process of learning — it lifts us all at once to infallible faith.
Throughout history, however, great minds have tried to find a fifth path to wisdom — one that's incomparably more certain and elevated than the other four. The road they attempted was the search for fundamental causes and true principles, from which we could work out the reasons behind everything a person can know. It's these thinkers, more than anyone, who have earned the title of "philosophers." And as far as I can tell, none of them has yet succeeded in this enterprise.
The first and most important among those whose writings survive are Plato and Aristotle. The only real difference between them was this: Plato, following in the footsteps of his teacher Socrates, honestly admitted that he had never been able to find anything certain. He was content to write about what seemed probable to him, inventing certain principles to try to explain everything else. Aristotle, on the other hand — though he was Plato's student for twenty years and had no principles beyond Plato's own — showed less honesty. He completely reversed the way he presented those principles and put forward as true and certain what he probably never really believed himself.
Both men had acquired great judgment and wisdom through the four means I described, which gave their authority enormous weight. The result was that those who came after them were more willing to accept their opinions than to search for better ones on their own. The main question debated among their followers, though, was whether we should doubt all things or hold some as certain. This dispute drove both sides into wild errors. Those who pushed doubt too far extended it even to everyday life, neglecting the most basic rules of practical conduct. Those who insisted on certainty, meanwhile, assumed it had to come from the senses and trusted them completely. Epicurus took this so far that he reportedly insisted — against all astronomical reasoning — that the sun is no bigger than it looks to the naked eye.
It's a pattern you can observe in most debates: truth lies somewhere in the middle between the two extreme positions, and each side moves away from it in proportion to how argumentative they are. The error of those who leaned too far toward doubt didn't last very long. The error of the opposite camp was corrected somewhat by the recognition that the senses deceive us in many cases. But I don't think this mistake was ever fully cleared up. It was never properly shown that certainty lies not in the senses but in the understanding alone — specifically, when the understanding has clear perceptions. At the same time, the point was missed that as long as we have only the knowledge acquired through those first four levels of wisdom, we shouldn't doubt what appears true in the conduct of everyday life. But we also shouldn't treat such things as so certain that we can never revise our opinions about them when reason compels us to.
Because this truth was either unknown or neglected, most of those who aspired to be philosophers in later centuries blindly followed Aristotle. They frequently distorted the meaning of his writings and attributed opinions to him that he wouldn't recognize as his own if he came back to the world today. And those who didn't follow him — among whom were many of the greatest minds — still couldn't escape being saturated with his ideas in their youth, since Aristotle's philosophy was the standard curriculum in the universities. Their minds were so thoroughly steeped in it that they couldn't rise to a knowledge of true principles.
I have great respect for all philosophers, and I don't want to make enemies with my criticism. But I can offer a proof of what I'm saying that I don't think any of them would deny. They all took as a starting principle something they didn't fully understand. For example, I know of none of them who didn't assume there was "gravity" in earthly bodies. Experience clearly shows us that objects we call "heavy" fall toward the center of the earth. But that doesn't mean we actually know the nature of gravity — that is, the cause or principle by which bodies fall. We'd have to figure that out from some other source. The same goes for the vacuum, atoms, heat and cold, dryness and moisture, salt, sulfur, mercury, and all the other things various philosophers adopted as their starting principles. But no conclusion drawn from an unclear principle can be truly evident, even if the logical reasoning is technically valid. It follows that no arguments built on such principles could ever lead to certain knowledge of anything, and consequently couldn't advance anyone a single step in the search for wisdom. And if these philosophers did discover any truth along the way, it was thanks to one or another of the four methods I mentioned earlier — not to their supposed principles.
I don't mean to diminish anyone's rightful reputation by saying this. I'm just pointing out something that should actually console people who haven't devoted themselves to formal study. It's like this: imagine you're traveling and you take a wrong turn, walking away from your destination. The longer you walk and the faster you go, the farther you get from where you want to be. Even if someone eventually puts you back on the right road, you still can't arrive as quickly as if you'd never gone the wrong way in the first place. Philosophy works the same way. When we start from false principles, we move farther from truth and wisdom in direct proportion to how carefully we develop those principles and how many conclusions we try to draw from them. We think we're doing great philosophy when we're actually heading in the wrong direction. The takeaway? Those who have learned the least of what has traditionally been called "philosophy" are actually the best prepared to grasp the truth.
Having explained all that, my next goal would have been to present the reasons for believing that the true principles I've laid out in this work are the ones that can lead us to the highest degree of wisdom — which is the supreme good of human life. Two considerations are enough to establish this. First, these principles are extremely clear. Second, everything else can be deduced from them. Those are the only two conditions true principles need to meet.
I can easily prove that they are clear. Consider how I discovered them: by rejecting every proposition that was even slightly doubtful. Whatever survived that test, when carefully examined, must be the most evident and clear ideas the human mind can possess. Specifically, by reflecting on the fact that anyone who tries to doubt everything still cannot doubt that they exist while they're doubting, and that the thing doing this reasoning — the thing that can't doubt itself even while doubting everything else — is not what we call the body, but what we call the mind or thought: from this I took the existence of thought as my first principle. From it I very clearly deduced the following truths: that there is a God who created everything in the world, and who, being the source of all truth, could not have given us an understanding that is systematically deceived when it forms judgments about things it perceives clearly and distinctly. These are all the principles I use when it comes to immaterial or metaphysical subjects. From them I deduce, with perfect clarity, the principles of physical things: namely, that there are bodies extended in length, width, and depth, which come in various shapes and move in various ways. Those, in brief, are the principles from which I derive all other truths.
The second thing that proves these principles are clear is that they've been known in every age, and accepted as true and beyond doubt by everyone — with the single exception of God's existence, which some people have doubted because they relied too heavily on the senses, and God can't be seen or touched.
But here's the thing: even though all the truths I've listed as my principles have been known at all times and by everyone, no one before me — as far as I know — has adopted them as the principles of philosophy. No one has used them as the foundation from which to deduce knowledge of everything else in the world. So what remains is for me to prove that they really can serve as that foundation. And the best way to prove this, I think, is through the test of experience — that is, by inviting readers to work through the book that follows. Even though I haven't covered every topic in it (that would be impossible), I believe I've explained the things I do discuss thoroughly enough that attentive readers will come away convinced: we don't need any principles beyond those I've provided to reach the most exalted knowledge the human mind is capable of. This should be especially clear if, after reading my work, they consider how many different questions it addresses, and then compare it to the writings of other philosophers to see how little plausibility there is in explanations built on different principles.
I could also point out that people who have absorbed my ideas find it much easier to understand and evaluate other philosophers' writings than people who haven't. This is exactly the opposite of what I said about those who started with ancient philosophy — that the more they studied it, the less capable they became of grasping the truth.
I'd also like to offer some advice about how to read this work. My suggestion is that the reader should first go through the whole thing the way you'd read a novel — without straining too hard, without getting stuck on any difficulties, just to get a general sense of what I'm discussing. Afterward, if the material seems worth a closer look and the reader wants to understand the reasons behind things, they should read it a second time, paying attention to how my arguments connect. But even then, they shouldn't despair if they can't always see the connections or follow every line of reasoning. Just mark the difficult passages with a pen and keep reading to the end. Then, if they're willing to pick up the book a third time, I'm confident a fresh reading will clear up most of the difficulties they'd noted before. And if any still remain, the solutions will eventually turn up in yet another reading.
I've noticed, from observing many different kinds of minds, that hardly anyone is so slow or dull that they're incapable of understanding sound ideas, or even of mastering the highest sciences — as long as they're guided along the right path. This can also be proved by reasoning: since the principles are clear, and since nothing should be deduced from them except by the most obvious inferences, no one lacks the intelligence to follow the conclusions that flow from them. The real obstacles are twofold. First, prejudices — from which no one is entirely free, though the people most harmed by them are those who studied false systems of thought most intensely. Second, there's a problem at both ends of the spectrum: people of average ability often give up because they've convinced themselves they're not smart enough, while more ambitious minds rush ahead too fast and end up adopting principles that aren't really self-evident and drawing shaky conclusions from them. So I want to reassure those who doubt their own abilities: there's nothing in my writings that you can't fully understand if you take the trouble to examine it carefully. And at the same time, I want to warn even the most brilliant minds that they'll need considerable time and attention to take in everything I've included.
After all that, to help people understand my real purpose in publishing these works, I'd like to explain the order I think a person should follow in educating themselves. First, someone who has only the ordinary, imperfect knowledge gained through the four means I described should, before anything else, develop a practical moral code — a set of rules sufficient for governing daily life. This is the top priority, partly because it can't wait, and partly because living well should always be our first concern. Next, they should study logic — not the logic taught in the universities, which is really just a method for explaining to others what you already know, or worse, for talking at length and without judgment about things you don't know. That kind of logic corrupts good sense rather than developing it. I mean the logic that teaches how to direct your reasoning properly in order to discover truths you don't yet know. Since this skill depends heavily on practice, it's a good idea to spend time applying its rules to easy, clear-cut problems — like those in mathematics. Once a person has developed some skill at finding truth in those kinds of questions, they should turn seriously to real philosophy.
The first part of philosophy is Metaphysics, which contains the principles of knowledge. This includes an account of God's principal attributes, the immateriality of the soul, and all the clear and simple ideas within us. The second part is Physics, where, after establishing the true principles of material things, we examine how the entire universe is structured. Then we look more closely at the nature of the earth and the bodies most commonly found on it — air, water, fire, magnets, and other minerals. After that, we need to study the nature of plants, animals, and above all human beings, so that we can eventually develop the other sciences that are useful to us.
Think of all of philosophy as a tree. Metaphysics is the roots, Physics is the trunk, and all the other sciences are the branches growing from that trunk. These branches reduce to three main ones: Medicine, Mechanics, and Ethics. By Ethics, I mean the highest and most complete kind of moral science — the one that presupposes a thorough knowledge of all the other sciences and represents the final stage of wisdom.
But just as we don't gather fruit from the roots or trunk of a tree, but only from the tips of its branches, the main practical benefit of philosophy comes from its individual applications — which happen to be the parts we learn last. Even though I'm still ignorant of most of these applications, my constant desire to be useful to the public led me to publish, about ten or twelve years ago, a series of essays on topics I thought I'd made progress in. The first part was the "Discourse on the Method of Rightly Conducting Reason and Seeking Truth in the Sciences," which summarized the main rules of logic and also an imperfect, provisional ethics — the kind a person can follow until they find something better. The remaining parts were three scientific treatises: one on Optics, one on Meteorology, and one on Geometry. In the Optics, I wanted to show that philosophy can take us far enough to arrive at practically useful knowledge, since the invention of the telescope (which I explained there) is one of the most difficult achievements ever. In the Meteorology, I wanted to highlight the difference between my philosophy and the kind taught in the universities, where the same topics are usually discussed. And in the Geometry, I aimed to demonstrate that I'd discovered many things previously unknown, giving people reason to believe many more discoveries are still possible — and hopefully inspiring everyone to join in the pursuit of truth.
Since then, anticipating that many readers would struggle with the foundations of my Metaphysics, I tried to explain the key points in a book of Meditations. The book itself isn't long, but it grew considerably in size thanks to the Objections that several very learned scholars sent me, along with my Replies. Finally, once it seemed to me that all these earlier works had adequately prepared my readers' minds, I published the Principles of Philosophy. I divided it into four parts. The first contains the principles of human knowledge and could be called "First Philosophy," or Metaphysics. To understand this part properly, it's helpful to read the Meditations first. The other three parts cover the most general topics in Physics: the fundamental laws of nature; how the heavens, the fixed stars, planets, comets, and the entire universe are structured; and then, in more detail, the nature of the earth, air, water, fire, magnets, and other bodies we commonly find around us, along with all the properties we observe in them — things like light, heat, and gravity.
In this way, I believe I've begun an orderly explanation of the whole of philosophy, without skipping any of the groundwork that should come before the topics I've discussed. But to complete this project, I would still need to explain, in the same systematic way, the nature of all the more specific bodies found on earth — minerals, plants, animals, and especially human beings. And after that, I'd need to give a thorough treatment of Medicine, Ethics, and Mechanics. That's what it would take to give the world a complete body of philosophy. I don't yet feel so old, or distrust my abilities so much, or consider myself so far from the knowledge that remains to be discovered, that I wouldn't dare attempt to finish this project — if I had the resources to carry out all the experiments needed to support and verify my reasoning. But seeing that this would require enormous expense, far beyond what a private individual like myself can afford without public support, and since I have no reason to expect such support, I believe I should be content going forward to study for my own benefit. Posterity will forgive me if I don't continue working on their behalf.
In the meantime, so you can see where I think I've already contributed to the common good, let me mention the fruits that can be gathered from my Principles.
The first fruit is the satisfaction the mind will find in discovering many truths it didn't know before. Although truth usually doesn't strike our imagination as powerfully as fiction and falsehood do — because truth tends to seem less spectacular and more straightforward — the gratification it provides is always deeper and more lasting.
The second fruit is that studying these principles will gradually train us to judge better about everything we encounter, making us wiser. This is exactly the opposite of what ordinary academic philosophy does, since we can easily see that the people we call "pedants" are actually worse at using their reason than they would have been if they'd never studied philosophy at all.
The third fruit is that the truths these principles contain, being highly clear and certain, will eliminate the grounds for pointless disputes and encourage people toward open-mindedness and harmony. This is the opposite of what happens in university debates, which gradually make participants more combative and opinionated — and are perhaps the main cause of the divisions and conflicts plaguing the world today.
The fourth and greatest fruit is that by developing these principles, people will be able to discover many truths I haven't uncovered myself, and so, advancing step by step from one discovery to the next, eventually acquire a complete knowledge of all philosophy and reach the highest degree of wisdom. Every art and craft, though rough and imperfect at the beginning, is gradually perfected through practice — as long as it starts from something true whose effects experience can confirm. The same goes for philosophy. When we have true principles, following them will inevitably lead us to further truths. And the best proof that Aristotle's principles are false? People made no progress in knowledge during all the many centuries they followed them.
I'm well aware that there are impulsive people with so little care for precision that even on the most solid foundations, they couldn't build a stable system. And since these are usually the people most eager to write books, they could quickly ruin everything I've done, introducing uncertainty and doubt into a way of philosophizing from which I've worked hard to banish them — if people were to mistake their writings for mine or take them to represent my views. I had an experience like this not long ago with someone widely believed to be one of my closest followers, and about whom I'd once said I had such confidence in his abilities that I thought he held no opinions I wouldn't be willing to claim as my own. But last year he published a book called "Fundamental Physics," in which — although he seems to have written nothing about physics or medicine that he didn't take from my works, both the published ones and an unfinished manuscript on the nature of animals that fell into his hands — he copied everything badly, rearranged the order, and rejected certain metaphysical truths on which all physics must be built. I'm therefore compelled to completely disavow his work, and I want to ask readers here not to attribute any opinion to me unless they find it explicitly stated in my own writings. And they should accept no opinion as true — whether in my works or anyone else's — unless they can see that it follows clearly from true principles.
I'm also well aware that many years may pass before all the truths that can be deduced from these principles are actually worked out. Most of the discoveries still to be made depend on specific experiments that don't happen by chance but require careful, expensive investigation by highly intelligent people. And it's unlikely that the same people who have the skill to use experiments properly will also have the resources to conduct them. On top of that, most of the best minds have developed such a low opinion of philosophy in general — because of the flaws they've noticed in the kind practiced up to now — that they can't bring themselves to pursue the search for truth.
But if the difference between my principles and those of every other system, together with the vast array of truths that can be deduced from them, leads people to recognize the importance of continuing this search — if they see the degree of wisdom, the fulfillment, and the quality of life these principles can lead us to — then I'm confident that no one will be unwilling to work hard at such a rewarding study, or at least to support and assist with all their strength those who devote themselves to it successfully.
My deepest wish is that future generations may someday see the happy outcome of this work.
Madam — The greatest benefit I have gained from the writings I have already published is that they brought me to your Highness's attention, and gave me the privilege of occasional conversation with someone who possesses so many rare and admirable qualities that I believe I would be doing the public a service by holding them up as an example for future generations.
It would be wrong of me to flatter, or to assert anything I did not know for certain — especially in the opening pages of a work devoted to establishing the principles of truth. And the generous modesty that is evident in everything you do assures me that the honest, straightforward judgment of a man who writes only what he believes will be more welcome to you than the elaborate praise of those who have mastered the art of empty compliments. For this reason, I will include nothing in this letter that I am not certain of through both experience and reason. Even in this dedication, as in the rest of the work, I will write only as a philosopher should.
There is a vast difference between real and apparent virtues, and also a great gap between real virtues that come from an accurate understanding of truth and those that are accompanied by ignorance or error.
What I call "apparent" virtues are really just vices in disguise. Because they are less common than the vices they oppose, and because they sit further from those vices than the genuinely virtuous middle ground, people tend to hold them in higher esteem than true virtues. For example, since people who fear danger too much outnumber those who fear it too little, recklessness gets treated as the opposite of cowardice and mistaken for a virtue — and it is usually valued more highly than true courage. In the same way, the recklessly generous get more praise than the genuinely generous, and no one builds a reputation for piety faster than the superstitious and the hypocritical.
As for true virtues, not all of them come from true knowledge. Some spring from deficiency or error: simplicity is often the source of goodness, fear the source of devotion, and despair the source of courage. Virtues that are mixed with such imperfections differ from one another and go by many different names.
But those pure, perfect virtues that arise solely from the knowledge of what is good are all essentially the same, and can all be gathered under a single word: wisdom. Anyone who holds a firm and constant resolution to always use their reason as well as they can, and to do what they judge to be best in all their actions — that person is truly wise, as far as their nature allows. And by this alone, they are just, courageous, and self-controlled, and possess all the other virtues, but in such balance that none stands out above the rest. For this reason, although these balanced virtues are far more perfect than the ones that shine brightly because they are mixed with some flaw, the crowd notices them less, and they rarely get the same applause.
Now, the wisdom I have just described requires two things: clear understanding and a well-directed will. The will is something all people can possess equally, even though some people's understanding is sharper than others'. So even those with more limited understanding can be perfectly wise — as far as their nature allows — and can make themselves highly pleasing to God through their virtue, provided they always maintain a firm resolution to do what they judge to be right, and never stop trying to learn about duties they do not yet understand.
Nevertheless, those who combine this constant resolution to do right with a special dedication to learning, and who also possess a particularly keen intellect, reach a higher degree of wisdom than anyone else. And I see all three of these qualities united, in great perfection, in your Highness.
First, your desire to educate yourself is obvious. Neither the amusements of the court nor the customary education of ladies — which typically condemns them to ignorance — has been enough to keep you from studying with great care everything that is best in the arts and sciences. Second, the extraordinary sharpness of your intellect is proven by the fact that you penetrated the heart of these subjects and gained a thorough command of them in a remarkably short time.
But I have an even stronger proof of the power of your mind — one that is personal to me. I have never yet met anyone who understood so thoroughly and so well everything contained in my writings. Many people, including some of the most brilliant and learned, find my work very obscure. And I have noticed that almost everyone who is skilled in metaphysics has no taste for geometry, while those who cultivate geometry have no ability for investigations in first philosophy. So I can honestly say that I know of only one mind — yours — to which both fields are equally natural. I have good reason, therefore, to call it incomparable.
But what amazes me most of all is that such an accurate and wide-ranging command of the entire circle of the sciences is found not in some aged scholar who has spent many years in study, but in a Princess still young — one whose appearance and age would more fittingly represent one of the Graces than a Muse, or the wise goddess Minerva.
In conclusion, I not only find in your Highness everything that is needed on the side of the mind for perfect and elevated wisdom, but also everything that can be asked on the side of the will and character. In you, kindness and gentleness are joined with dignity in such a way that, although fortune has attacked you with relentless injustice, it has failed either to embitter or to crush you. This compels such deep respect in me that I not only consider this work your due — since it is a work of philosophy, the study of wisdom — but I also take as much pleasure in signing myself your devoted servant as I take pride in my reputation as a philosopher.
Since we were once children who formed all sorts of judgments about the things we perceived with our senses — before we had full use of our reason — a great many prejudices stand in the way of our arriving at the truth. The only way to free ourselves of these prejudices, it seems, is to undertake, at least once in our lives, to doubt everything in which we can find even the smallest ground for uncertainty.
It will also be useful to treat as false anything we are able to doubt. This way, we can more clearly discover what is truly most certain and easiest to know.
In the meantime, it should be noted that this kind of sweeping doubt is only meant for use when we are seeking the truth through contemplation. In everyday life, we are very often forced to act on opinions that are merely probable. Sometimes we must choose between two courses of action even when neither one seems more likely than the other, since if we waited to resolve all our doubts, the opportunity to act would often pass us by.
With this in mind, since we are now focused solely on investigating the truth, we will begin by doubting whether any of the things we have ever perceived through our senses, or imagined, actually exist. First, because experience shows us that the senses sometimes deceive us, and it would be unwise to fully trust what has even once misled us. Second, because in our dreams we constantly seem to perceive or imagine countless things that have no real existence. And once someone has committed to this kind of thorough doubt, there seem to be no reliable marks by which to distinguish sleep from waking.
We will also doubt the things we previously held as most certain — even mathematical proofs and the principles we considered self-evident. First, because we have sometimes seen people make mistakes in such matters, accepting as absolutely certain and self-evident what turned out to be false. But the main reason is this: we have learned that God, who created us, is all-powerful. We do not yet know whether he might have chosen to make us in such a way that we are always deceived, even in what we think we know best. This does not seem any less possible than the occasional deception we already know from experience. And if we suppose that no all-powerful God is the author of our existence — that we exist by ourselves or by some other means — then the less powerful we suppose our maker to be, the more reason we have to think we are imperfect enough to be continually deceived.
But no matter who may ultimately be the author of our existence, and no matter how powerful or deceptive that being might be, we are still conscious of a freedom within ourselves. We can always choose to withhold our belief from anything that is not manifestly certain and beyond doubt, and in this way guard ourselves against ever being deceived.
When we reject everything we can entertain even the smallest doubt about — even going so far as to imagine it is all false — we can easily suppose that there is no God, no sky, no physical objects, and that we ourselves have neither hands nor feet nor any body at all. But we cannot in the same way suppose that we do not exist while we are doubting these things. There is a contradiction in thinking that what thinks does not exist at the very moment it is thinking. And so the knowledge "I think, therefore I am" is the first and most certain truth that anyone arrives at when they philosophize in an orderly way.
This is also the best way to discover the nature of the mind and how it differs from the body. When we ask what we are — while supposing, as we now do, that nothing exists apart from our thinking — we clearly see that neither extension, nor shape, nor motion in space, nor anything else that can be attributed to a body belongs to our nature. Nothing belongs to us except thought alone. It follows that our concept of the mind comes before our concept of anything physical, and is more certain. After all, we are still doubting whether any body exists, yet we already know that we think.
By the word "thought," I mean everything that takes place in us in such a way that we are immediately aware of it. Accordingly, not only understanding, willing, and imagining, but even perceiving through the senses counts here as thinking. For if I say "I see" or "I walk," and therefore I exist — and if by "seeing" or "walking" I mean the physical act of my eyes or my legs, which is a bodily function — then the conclusion is not absolutely certain. As often happens in dreams, I might think I see or walk even though I have not opened my eyes or moved from my spot, and perhaps even though I have no body at all. But if I mean the sensation itself — the consciousness of seeing or walking — then the knowledge is clearly certain, because it refers to the mind, which alone perceives or is conscious that it sees or walks.
I am not going to define several other terms I have used or plan to use later, because their meaning seems sufficiently obvious. I have often noticed that philosophers make a mistake when they try to explain the most simple and self-evident truths through logical definitions — they only end up making them more obscure. And when I said that the proposition "I think, therefore I am" is the first and most certain truth that anyone reaches through orderly philosophizing, I did not mean to deny that you first need to know what thought, existence, and certainty are, or that in order to think you must exist, and so on. But because these are the simplest notions of all — and by themselves they do not tell us that anything exists — I did not think it was appropriate to formally list them.
Now, to see how our knowledge of the mind is not only prior to and more certain than our knowledge of the body, but even clearer, we should note something that is plainly obvious by the natural light of reason: nothing has properties or qualities if it is nothing at all. Wherever we observe certain properties, there must also be a thing or substance to which those properties belong. The same natural light shows us that we know a thing or substance more clearly the more properties we discover in it.
Now, we recognize a greater number of properties in our mind than in anything else. This is because whenever we come to know anything at all, we are at the same time — and with even greater certainty — led to knowledge of our own mind. For example, if I judge that the earth exists because I touch or see it, then by that very same reasoning, and with even stronger grounds, I must conclude that my mind exists. It is possible that I am wrong about touching the earth, and that no earth actually exists. But it is not possible for me to make that judgment without my mind — the thing doing the judging — existing. The same holds for anything whatsoever that is presented to our mind.
People who have not philosophized in an orderly way have held different views on this, because they never carefully distinguished the mind from the body. Although they had no trouble believing that they themselves existed — and were more certain of this than of anything else — they failed to notice that by "themselves" they should have meant their minds alone, at least when the question concerned metaphysical certainty. Instead, they meant their bodies — which they saw with their eyes and touched with their hands — and to which they mistakenly attributed the power of perception. This prevented them from clearly grasping the nature of the mind.
But when the mind knows itself yet remains in doubt about everything else, it looks around on all sides to extend its knowledge further. First, it discovers within itself the ideas of many things. As long as it simply contemplates these ideas — neither affirming nor denying that anything outside itself corresponds to them — it is in no danger of error. The mind also discovers certain common principles from which it constructs various proofs so convincing that doubt seems impossible, at least while we are paying attention to them. For example, the mind contains ideas of numbers and shapes, and among its common principles is the rule that "if equals are added to equals, the results are equal," and others like it. From these, it is easy to prove that the three angles of a triangle are equal to two right angles, and so on.
As long as we focus on the premises from which these conclusions follow, we feel confident they are true. But the mind cannot always stay focused on them. When it recalls a conclusion without remembering the steps of the proof, and is still unsure whether its creator made it the kind of thing that might be deceived even about what seems most obvious — then it has legitimate grounds for doubting such conclusions. It cannot achieve certain knowledge until it has discovered who made it.
When the mind then reviews the various ideas it contains, it discovers one that stands out above all the rest: the idea of a Being who is all-knowing, all-powerful, and absolutely perfect. In this idea, the mind notices something remarkable. Unlike the ideas of all other things, which contain only possible or contingent existence, this idea contains necessary and eternal existence. Just as the equality of its three angles to two right angles is necessarily contained in the idea of a triangle — making the mind fully convinced that a triangle's angles equal two right angles — so too, necessary and eternal existence is contained in the idea of an all-perfect Being. From this, the mind should clearly conclude that this all-perfect Being exists.
The mind will be even more certain of this if it considers that it has no idea of any other thing in which necessary existence is contained. From this alone, it will see that the idea of an all-perfect Being was not fabricated by the mind itself and does not represent something imaginary. Rather, it represents a true and unchangeable nature that must exist, since it can only be conceived as necessarily existing.
Our mind would have no trouble accepting this truth if it were completely free from prejudices. But since we are used to distinguishing essence from existence in everything else, and since we can freely invent ideas of things that neither exist nor have ever existed, it easily happens — when we do not keep our thoughts firmly focused on the all-perfect Being — that we start to wonder whether the idea we have of God is one we made up, or at least one of those ideas whose essence does not include existence.
When we reflect further on the various ideas within us, it is easy to see that, considered simply as ways of thinking, they do not differ much from one another. But considered in terms of the objects they represent, they differ enormously. And their causes must be proportionally more perfect according to the degree of objective perfection the ideas contain. This is no different from the case of someone who has the idea of a machine of extraordinary ingenuity. We have every right to ask where this idea came from — whether, for example, he saw such a machine built by someone else, or was so thoroughly taught the mechanical sciences, or possesses such natural genius that he could invent it on his own without ever seeing anything like it. For all the ingenuity contained in the idea — as if in a picture — must exist at least in its first and primary cause, not merely as a representation, but truly and actually.
Since we find in our minds the idea of God — an all-perfect Being — we have the right to ask where we got it. And we will discover that the perfections it represents are so vast that we could only have received it from an all-perfect Being: that is, from a God who truly exists. For it is obvious by the natural light that nothing can come from nothing, and that the more perfect cannot be produced by the less perfect as its complete cause. Moreover, we cannot have the idea or representation of anything unless there is somewhere — either in us or outside us — an original that actually contains all the perfections that idea represents. Since we do not find in ourselves the absolute perfections of which we have this idea, we must conclude that they exist in some nature different from ours — namely, in God. Or at least they were once in him. And it most clearly follows from their infinity that they are still there.
This will be sufficiently clear to anyone who is used to reflecting on the idea of God and considering his infinite perfections. Although we cannot fully comprehend them — because it is in the nature of the infinite not to be comprehended by what is finite — we nevertheless conceive them more clearly and distinctly than we do material objects. This is because God's perfections, being simple and unlimited, fill our mind more completely.
Not everyone has noticed this, however. And because when we have the idea of some ingenious machine, we usually know perfectly well how we came by that idea — yet we cannot even remember when the idea of God was first communicated to us, since it has always been in our minds — we must continue our inquiry and ask about the source of our own being. After all, we possess the idea of a God with infinite perfections. It is perfectly evident by the natural light that anything which knows something more perfect than itself cannot be the source of its own existence. If it were, it would have given itself all the perfections it knows about. Therefore, it can only derive its existence from a being that actually possesses all those perfections — that is, from God.
The truth of this argument becomes clear when we consider the nature of time. The parts of time do not depend on one another and never exist simultaneously. So from the fact that we exist right now, it does not necessarily follow that we will exist a moment from now — unless some cause, the same one that first produced us, continually reproduces us, so to speak, or in other words, keeps us in existence. For we easily understand that we have no power in ourselves to sustain our own existence. And the being that has enough power to sustain us must, by even greater reason, sustain itself — or rather, need no sustaining at all — and thus, in short, be God.
There is a great advantage in proving God's existence in this way — namely, through his idea. It allows us to know at the same time what God is, as far as our limited nature permits. For when we reflect on the innate idea we have of him, we see that he is eternal, all-knowing, all-powerful, the source of all goodness and truth, the creator of all things, and that he possesses in himself everything in which we can clearly recognize infinite perfection or goodness that is not limited by any imperfection.
There are indeed many things in the world that are to some degree imperfect or limited while also possessing some perfection. None of these imperfections can be in God. Thus, since divisibility is included in physical extension, and divisibility indicates imperfection, it is certain that God is not a body. And although it is in some sense a perfection in humans to be able to perceive through the senses, every sense involves passivity, which indicates dependence. So we must conclude that God does not perceive through senses in any way. He only understands and wills — but not as we do, through separate acts. Rather, he always acts through a single, identical, and utterly simple act by which he understands, wills, and brings about everything that truly exists. He does not will the evil of sin, since that is merely the absence of being.
Now that we know God alone is the true cause of everything that exists or can exist, the best way to philosophize will be to start from our knowledge of God himself and try to explain the things he has created. We should try to deduce this knowledge from the ideas naturally present in our minds, for this will give us the most perfect kind of science: knowledge of effects through their causes. But to do this without falling into error, we must be careful to keep in mind that God, the author of all things, is infinite, while we are entirely finite.
So if God should reveal to us or to others matters about himself that surpass the natural powers of our mind — such as the mysteries of the incarnation and the trinity — we will not refuse to believe them, even though we may not clearly understand them. Nor will we be surprised to find, in the immensity of his nature or even in what he has created, many things that exceed our comprehension.
We will therefore never get tangled up in debates about the infinite. It would be absurd for us, being finite, to try to determine anything about it, and thus to try to limit it by attempting to comprehend it. So we will not bother responding to those who ask whether half of an infinite line is also infinite, or whether an infinite number is even or odd, and so on. Only those who imagine their minds to be infinite seem obligated to entertain questions like that.
For our part, when we find things that seem to have no limits in certain respects, we will not call them infinite but will simply regard them as indefinite. So because we cannot imagine an extension so large that we cannot conceive of a larger one, we will say that the magnitude of possible things is indefinite. Because a body can always be divided into parts that are themselves divisible into yet smaller parts, we will regard quantity as divisible into an indefinite number of parts. Because we cannot imagine so many stars that God could not create still more, we will suppose their number is indefinite. And so on for other cases.
We will call these things "indefinite" rather than "infinite" in order to reserve the term "infinite" for God alone. First, because in God we not only discover no limits in any respect, but we also positively conceive that he has none. Second, because with other things, we do not in the same positive way conceive that they are unlimited in every respect. We merely recognize, in a negative way, that if they have any limits, we cannot discover them.
Likewise, we will not seek to explain natural things by asking what purpose God or nature intended in creating them — that is, we will not look for final causes. We should not presume to think we can read the mind of God. Instead, considering God as the efficient cause of all things, let us use the natural light of reason he has given us — applied to those of his attributes he has been willing for us to know — to work out what must follow regarding the effects we perceive through our senses. We should keep in mind, however, as already noted, that we can only trust this natural light as long as nothing contrary to its findings is revealed by God himself.
The first of God's attributes we should consider here is that he is absolutely truthful and the source of all light. It is therefore plainly contradictory for him to deceive us, or to be properly and directly the cause of the errors we experience. For although the ability to deceive might seem like a sign of cleverness among humans, the desire to deceive always stems from malice, fear, or weakness — none of which can be attributed to God.
From this it follows that the natural light — the faculty of knowledge God gave us — can never grasp an object incorrectly, insofar as it truly grasps it: that is, insofar as the object is clearly and distinctly perceived. For God would deserve to be called a deceiver if he had given us a faculty that was defective in such a way that it led us to mistake falsehood for truth when we used it properly. This removes the most serious doubt we raised earlier — the worry that perhaps our nature itself was so flawed that we might be deceived even in what seems most obvious.
The same principle should also dispel all the other grounds for doubt previously mentioned. Mathematical truths should now be beyond suspicion, since they are among the clearest truths we have. And if we perceive anything through our senses, whether awake or asleep, we can easily discover the truth as long as we separate what is clear and distinct in our perception from what is obscure and confused. I will not say more about this here, since it was already thoroughly treated in the Meditations, and what follows will explain it even more precisely.
Although God does not deceive us, we still frequently fall into error. If we want to understand where our errors come from, in order to guard against them, we need to recognize that they depend less on our understanding than on our will. They do not require any active contribution from God to be produced. When considered in relation to God, our errors are merely negations — the absence of something. But in relation to ourselves, they are privations — the lack of something we ought to have.
All the modes of thinking we are conscious of can be grouped into two general classes. The first is perception, which is the operation of the understanding. The second is volition, which is the operation of the will. Perceiving through the senses, imagining, and conceiving things that are purely intellectual are all different forms of perceiving. Desiring, being averse to something, affirming, denying, and doubting are all different forms of willing.
When we perceive something, we are in no danger of error as long as we refrain from making any judgment about it. Even when we do form a judgment, we would never go wrong if we only gave our assent to what we clearly and distinctly perceived. The reason we usually get deceived is that we make judgments without having adequate knowledge of what we are judging.
I grant that the understanding is necessary for judging, since we obviously cannot judge what we do not perceive at all. But the will is also required, since we must choose to give our assent to what we have perceived. It is not necessary, however, to have a complete and perfect understanding of something in order to judge it. We often assent to things of which we have only a very vague and confused grasp.
Furthermore, the reach of the intellect extends only to the few things presented to it, and it is always very limited. The will, on the other hand, can in a sense be called infinite, because we observe that there is nothing that could be the object of any other will — even the unlimited will of God — that our will cannot also reach toward. So we easily extend our will beyond what we clearly perceive, and when we do, it is no wonder that we end up being deceived.
But although God has not given us an all-knowing intellect, this does not make him in any way the author of our errors. It is simply in the nature of a created intellect to be finite, and in the nature of a finite intellect not to grasp everything.
The fact that the will is broader than the understanding actually fits its nature perfectly. It is a great perfection in human beings to be able to act freely, and it is precisely this freedom that makes us responsible for our actions and worthy of praise or blame. We do not praise self-operating machines for performing their movements exactly as designed, since they act out of necessity. Instead, we praise the maker for the skill with which he built them, because he acted freely. On the same principle, we deserve credit when we embrace the truth, precisely because we do so by choice, not by compulsion.
It is true that whenever we err, there is some defect in how we act or how we use our freedom — but not in our nature, which remains the same whether our judgments are true or false. And although God could have given us such a sharp intellect that we would never have made a mistake, we have no right to demand this of him. Among humans, a person who could have prevented someone else's wrongdoing but did not is held responsible, because human authority exists precisely for the purpose of keeping people from doing wrong. But God's dominion over the universe is absolutely and completely free. For this reason, we should thank him for the good things he has given us, rather than complaining that he did not give us everything we know he could have.
That we possess free will — the power to give or withhold our assent — is so obvious that it must be counted among the first and most basic truths we are born knowing. This has already become very clear: when we set out to doubt everything, and even supposed that our all-powerful creator was deceiving us in every possible way, we were still conscious of being free to withhold belief from anything that was not completely certain and beyond doubt. And whatever we cannot doubt even in those extreme circumstances is as self-evident and certain as anything we can ever know.
But because what we have already discovered about God assures us that his power is so immense that we would be wrong to think ourselves capable of ever doing anything he had not ordained in advance, we would quickly run into serious difficulties if we tried to reconcile God's preordination with the freedom of our will and grasp both truths at once.
Instead of getting entangled in those difficulties, we should remember that our mind is limited while the power of God — by which he not only knew from all eternity everything that is or can be, but also willed and preordained it — is infinite. We have enough intelligence to know clearly and distinctly that this power exists in God, but not enough to comprehend how he leaves the free actions of human beings undetermined. On the other hand, we have such a vivid awareness of the freedom and openness that exists in ourselves that there is nothing we know more clearly or completely. The omnipotence of God should not make us doubt this. It would be absurd to doubt what we experience directly in ourselves just because we cannot fully grasp another matter that we know, by its very nature, to be beyond our comprehension.
Now that we know all our errors depend on our will, and that no one wishes to deceive himself, it may seem puzzling that we make any errors at all. But we need to distinguish between wanting to be deceived and wanting to assent to opinions that happen to contain error. No one deliberately chooses to be wrong. Yet hardly anyone hesitates to assent to things in which error lurks without their knowing it. In fact, it is often the very desire to find the truth that leads people — when they do not fully understand the proper method for seeking it — to rush to judgment on matters they do not adequately understand, and thus to fall into error.
But this much is certain: we will never mistake falsehood for truth as long as we judge only what we clearly and distinctly perceive. Since God is no deceiver, the faculty of knowledge he has given us cannot be deceptive, nor can our faculty of will lead us astray when we do not extend it beyond what we clearly know. Even if this truth could not be established by reasoning, the minds of all people have been so constituted by nature as to spontaneously assent to whatever is clearly perceived, and to find it impossible to doubt.
It is equally certain that when we accept a conclusion we do not really understand, we are either mistaken or, if we happen to stumble on the truth, it is only by luck. Either way, we can never be confident we are not in error. I admit it rarely happens that we judge something we have noticed we do not understand, because it is a dictate of the natural light never to judge what we do not know. But our most frequent mistake is this: we assume we have a past knowledge of many things, giving our assent to them as if they were stored reliably in memory and perfectly understood — when in truth, we have no such knowledge.
There are indeed a great many people who go through their entire lives without ever perceiving anything in the way required for sound judgment. The knowledge on which we can build certain and indubitable judgment must be not only clear but also distinct. I call a perception "clear" when it is present and open to the attentive mind — just as we say we see objects clearly when they are right in front of our eyes, stimulating them with enough force, and our eyes are focused on them. I call a perception "distinct" when it is so precise and sharply separated from every other perception that it contains within itself nothing but what is clear.
For example, when someone feels intense pain, the awareness they have of that pain is very clear. But it is not always distinct, because people usually confuse it with a vague judgment they form about the nature of the pain. They assume that something in the painful body part resembles the sensation of pain they experience. In this way, a perception can be clear without being distinct. But it can never be distinct without also being clear.
Indeed, in our early years the mind was so immersed in the body that, although it perceived many things clearly enough, it never knew anything distinctly. And since we already exercised our judgment on many things at that age, we picked up numerous prejudices that most people never manage to shed. To put ourselves in a position to get rid of these, I will briefly list all the simple notions that make up our thoughts, and distinguish in each what is clear from what is obscure or likely to lead us into error.
Whatever objects fall under our knowledge, we consider them either as things (or properties of things), or as eternal truths that have no existence outside our thought. Among things, the most general notions are substance, duration, order, number, and perhaps a few others that apply to every kind of thing. However, I recognize only two highest categories of things. The first is intellectual things — things that have the power to think — which includes mind or thinking substance and its properties. The second is material things — extended substance, or body, and its properties. Perception, volition, and all the various modes of knowing and willing relate to thinking substance. To extended substance we attribute magnitude (that is, extension in length, breadth, and depth), shape, motion, position, divisibility of parts, and the like.
There are also, however, certain things we experience internally that should not be attributed to the mind alone or to the body alone, but to the close and intimate union between them, as will be explained in its proper place. These include the appetites of hunger and thirst, and also the emotions or passions of the mind that are not purely mental — such as anger, joy, sadness, and love. Finally, there are all the sensations: pain, pleasure, light and colors, sounds, smells, tastes, heat, hardness, and the other qualities we perceive by touch.
What I have just listed, we should regard as things, or the qualities or modes of things. Now we come to eternal truths. When we recognize that something cannot come from nothing, the proposition "nothing comes from nothing" is not treated as a thing or as the property of a thing, but as an eternal truth that resides in our mind — what we call a common notion or axiom. Other examples include: the same thing cannot both be and not be at the same time; what is done cannot be undone; whoever thinks must exist while they think; and countless others. It would be difficult to list them all, but this is not necessary, since as long as we are not blinded by prejudices, we cannot fail to recognize them when the occasion for thinking about them arises.
As for these common notions, there is no doubt that they can be clearly and distinctly known — otherwise they would not deserve the name. Yet some of them are not equally accepted by everyone. This is not because some people have a broader faculty of knowledge than others, but rather because these common notions sometimes conflict with certain people's prejudices. Those who hold such prejudices cannot easily embrace these truths, even though others who are free from those prejudices grasp them with perfect clarity.
Now, regarding what we consider as things or the modes of things, it is worth examining each concept on its own. By "substance," we can only mean a thing that exists in such a way that it needs nothing else in order to exist. Strictly speaking, there can be only one substance that is absolutely independent, and that is God. All other things can only exist with the ongoing support of God. Accordingly, the term "substance" does not apply to God and to created things in the same sense — to use a term familiar from academic philosophy, it is not used "univocally." No single meaning of this word can be clearly understood as applying to both God and creatures.
Created substances, however — whether physical or thinking — can be understood under a common concept. They are things that need nothing but God's ongoing support in order to exist. But substance itself cannot be first discovered merely from the fact that something exists independently, since independent existence is not something we directly observe. We can, however, easily discover substance from any of its properties, based on the common principle that nothing has no properties. When we perceive that some property is present, we can infer that some existing thing or substance to which it belongs must also be present.
Although any property is sufficient to lead us to the knowledge of a substance, there is one principal property of every substance that constitutes its nature or essence, and on which all the others depend. Extension in length, breadth, and depth constitutes the nature of physical substance, and thought constitutes the nature of thinking substance. Everything else that can be attributed to body presupposes extension and is merely some mode of an extended thing. Likewise, all the properties we discover in the mind are merely different modes of thinking. For example, we cannot conceive shape without conceiving something extended, nor motion without extended space, nor imagination or sensation or will without a thinking thing. But we can conceive extension without shape or motion, and thought without imagination or sensation, and so on — as is clear to anyone who reflects on these matters.
In this way, we can easily form two clear and distinct ideas: one of created thinking substance, the other of physical substance — provided we carefully distinguish all the properties of thought from those of extension. We can also form a clear and distinct idea of an uncreated and independent thinking substance — that is, of God — provided we do not suppose this idea fully represents everything that is in God, and do not mix anything fictitious into it. Instead, we should attend simply to what is contained in our concept of him and what we clearly know belongs to the nature of an absolutely perfect Being. For no one can deny that we have such an idea of God, unless they groundlessly assume there is no knowledge of God at all in the human mind.
We will also have very clear conceptions of duration, order, and number if, instead of mixing in what properly belongs to the concept of substance, we simply think of the duration of a thing as a way of conceiving it insofar as it continues to exist. In the same way, order and number are not things distinct from the things arranged in order or counted, but simply ways we think about those things.
What we are calling "modes" here is the same as what we elsewhere call "attributes" or "qualities." But when we consider a substance as affected or varied by them, we use the term "modes." When these variations allow the substance to be classified as a certain kind, we use the term "qualities." And when we simply regard them as belonging to the substance, we call them "attributes." Accordingly, since God should be conceived as beyond change, it is not proper to say there are modes or qualities in God, but only attributes. And even in created things, whatever is always found in them in the same way — such as existence and duration in a thing that exists and endures — should be called an attribute rather than a mode or quality.
Some of these attributes or modes exist in the things themselves, while others exist only in our thought. Take time, for example, which we distinguish from duration in general and call the measure of motion. Time is really just a way we think about duration itself. We do not actually conceive the duration of things that move as being different from the duration of things that do not move. This is clear from the fact that if two bodies are in motion for an hour — one moving quickly and the other slowly — we do not count more time for one than for the other, even though there is much more motion in one body than in the other. But to measure the duration of all things by a common standard, we compare their duration to the largest and most regular motions that produce years and days, and we call this "time." So what we call time is nothing added to duration in general — it is simply a way of thinking about it.
In the same way, number, when it is not considered as existing in created things but merely in the abstract, is only a way of thinking. The same is true of all those general ideas we call universals.
Universals arise simply from our using one and the same idea to think about all the individual things that share a certain resemblance. When we group all the objects represented by this idea under one name, that name also becomes universal. For example, when we see two stones and pay no attention to their nature beyond noting that there are two of them, we form the idea of a certain number, which we call "two." When we later see two birds or two trees and merely notice there are two of them, we take up the same idea as before, which is therefore universal. And we likewise give this number the same universal name "two."
In the same way, when we consider a figure with three sides, we form an idea we call "triangle" and then use it as a universal to represent all other three-sided figures. But when we notice that some three-sided figures have a right angle and others do not, we form the universal idea of a right-angled triangle. This, being more specific than the previous idea, can be called a species, with the right angle being the universal difference that distinguishes right-angled triangles from all others. Further, because the square of the side opposite the right angle equals the sum of the squares of the other two sides, and because this property belongs only to this species of triangle, we can call it the universal property of the species. Finally, if we notice that some of these triangles are in motion and others are not, this will be their universal accident. Accordingly, there are traditionally five universals: genus, species, difference, property, and accident.
But number in things themselves arises from the distinctions between them. Distinction comes in three kinds: real, modal, and of reason. A real distinction properly holds between two or more substances. To be certain that two substances are really distinct from each other, it is enough that we can clearly and distinctly conceive one without the other. For our knowledge of God assures us that he can bring about anything of which we have a distinct idea. So since we have, for example, the idea of an extended, physical substance — even though we do not yet know for certain whether any such thing actually exists — the mere fact that we have this idea is enough to assure us that it could exist. And if it does exist, every part of it that we can distinguish in thought must be really distinct from every other part.
In the same way, since everyone is conscious that they think, and can in thought exclude from themselves every other substance — whether thinking or extended — it is certain that each of us, so considered, is really distinct from every other thinking and physical substance. And even if we suppose that God has united a body to a soul as closely as possible, making them into a single composite whole, the two substances would remain really distinct despite this union. No matter how tightly God might have joined them, he could not have given up the power to separate them or to preserve one apart from the other. And whatever God can separate or preserve separately is really distinct.
There are two kinds of modal distinction. The first is between a mode and the substance it belongs to. The second is between two modes of the same substance.
We recognize the first kind when we can clearly grasp a substance apart from a particular mode, but cannot conceive the mode without conceiving the substance. For example, there is a modal distinction between shape or motion and the physical substance in which they exist, and a similar distinction between affirmation or memory and the mind.
We recognize the second kind when we can think of one mode without the other, though we cannot think of either without thinking of the substance they both belong to. For example, if a stone is square and moving, we can conceive its square shape without its motion, and its motion without its square shape. But we cannot conceive either the motion or the shape apart from the substance of the stone.
As for a mode of one substance being different from another substance, or from a mode of another substance — such as the motion of one body being different from another body, or from the mind, or motion being different from doubt — this should be called a real distinction rather than a modal one, because these modes cannot be clearly conceived apart from the really distinct substances to which they belong.
Finally, the distinction of reason is the distinction between a substance and one of its attributes — an attribute without which, however, we cannot have a distinct conception of the substance — or between two such attributes of the same substance, where we try to think of one without the other. This distinction becomes apparent from our inability to form a clear and distinct idea of the substance if we separate that attribute from it, or to clearly perceive one such attribute if we separate it from the other. For example, because any substance that ceases to endure also ceases to exist, duration is distinct from substance only in thought. And in general, all the ways of thinking by which we consider objects differ only in thought — both from the objects they are thought about, and from each other when applied to the same object.
I should note that I believe I once classified this kind of distinction with the modal distinction — namely, toward the end of my Reply to the First Objections to the Meditations. But that was only necessary to treat these distinctions in a general way, and for my purposes at the time it was enough simply to distinguish both of them from the real distinction.
Thought and extension can be regarded as constituting the natures of thinking and physical substance respectively. In that case, they should be conceived as nothing other than thinking and extended substance themselves — that is, as mind and body. Understood this way, they are conceived with the greatest clarity and distinctness. Moreover, we more easily conceive thinking substance or extended substance than substance by itself — that is, with its thinking or extension left out. There is some difficulty in abstracting the notion of substance from the notions of thinking and extension, which are only different from each other in thought. A concept does not become more distinct just because it contains fewer elements; it becomes more distinct when we accurately distinguish what it contains from everything else.
Thought and extension can also be considered as modes of substance, insofar as one and the same mind can have many different thoughts, and one and the same body, keeping its size unchanged, can be extended in different ways — sometimes more in length and less in breadth or depth, other times more in breadth and less in length. In this case, they are modally distinguished from substance and can be conceived just as clearly and distinctly, provided they are not treated as substances or independent things, but simply as modes of things. By regarding them as belonging to the substances they are modes of, we distinguish them properly and see them for what they truly are. If, on the other hand, we tried to consider them apart from the substances they belong to, we would be treating them as self-standing things and would confuse the ideas of mode and substance.
In the same way, we will best understand the various modes of thought — such as understanding, imagining, remembering, willing, and so on — and the various modes of extension — such as all shapes, the arrangement of parts, and their motions — provided we consider them simply as modes of the things they belong to. As for motion in particular, we will understand it well enough if we think only of local motion — movement from place to place — without trying to account for the force that produces it, which I will attempt to explain in its proper place.
There remain our sensations, emotions, and appetites, which we can also know clearly — as long as we are careful to include in our judgments only what is precisely contained in our perception and of which we are directly aware. But this is very difficult to do, at least when it comes to sensations. The reason is that all of us, from our earliest years, have judged that everything we perceived through our senses had an existence outside our minds and was entirely similar to our sensations — that is, to our perceptions of them. So when we saw a certain color, for instance, we thought we were seeing something in the outside world that was identical to the idea of color we were experiencing. And from the habit of judging this way, we came to regard the externality of the color as so obvious that we considered it certain and indubitable.
The same prejudice applies to all our other sensations, including pleasure and pain. Even though we do not usually believe that objects resembling pleasure and pain exist outside us, we still do not consider these sensations to be in the mind alone or in our perception. Instead, we locate them in the hand, or the foot, or some other part of the body. But there is no reason to believe that the pain we feel "in" the foot, for example, is something outside the mind that actually exists in the foot, or that the light we seem to see "in" the sun exists in the sun the way it does in us. Both of these beliefs are prejudices from our childhood, as will become clear later.
To separate what is clear in our sensations from what is obscure, we must carefully observe the following. We have a clear and distinct knowledge of pain, color, and similar things when we consider them simply as sensations or thoughts. But when we judge them to be real things existing outside our minds, we are completely unable to form any conception of what they could be. Indeed, when someone says they see a color in a body or feel pain in a limb, it is exactly as if they said they see or feel something there whose nature they are entirely ignorant of — or in other words, that they do not know what they see or feel.
Admittedly, a person who does not examine their thoughts carefully may easily convince themselves they have some knowledge of what the sensation is, since they assume there is something in the external world resembling the sensation of color or pain they are experiencing. But if they actually reflect on what the sensation of color or pain represents as existing in the colored body or wounded limb, they will find they have absolutely no knowledge of it.
This will become even more obvious if we consider the following. Size, shape, motion (at least local motion — philosophers have made other kinds of motion less intelligible by inventing them), the arrangement of parts, duration, number, and those other properties that we clearly perceive in all bodies — these are known in a completely different way from how we know color, pain, smell, taste, or any of the other properties that, as I said above, must be attributed to the senses. For although when we see a body we are no less certain of its existence from its having a shape than from its having a color, we know its shape far more clearly than its color.
It is therefore clear that saying we perceive colors in objects is really the same as saying we perceive something in objects whose nature we do not know — except that it produces in us a certain very vivid and clear sensation, which we call the sensation of color. But there is a very important difference in how we judge this. As long as we simply judge that there is something unknown in objects — in things as they actually are — from which the sensation reaches us, we are far from falling into error. In fact, we are guarding against it, because recognizing that we do not know what something is makes us less likely to judge rashly.
But when we think we actually perceive colors in objects — even though we are really ignorant of what we are calling "color" and cannot conceive any resemblance between the color we suppose to be in objects and what we experience in sensation — we fail to notice this. And because there are other properties of objects, such as size, shape, and number, that either exist or can exist in them exactly as our senses perceive or our understanding conceives them, we easily slide into the error of thinking that what we call color in objects is something just like the color we perceive. We end up supposing we clearly perceive something that we do not perceive at all.
Here we can identify the first and chief cause of our errors. In early life, the mind was so closely tied to the body that it paid attention to nothing beyond the thoughts caused by objects affecting the body. It did not yet relate these thoughts to anything existing outside itself. It simply felt pain when the body was hurt, pleasure when something beneficial happened, and when the body was affected in ways that neither greatly helped nor harmed it, the mind experienced what we call tastes, smells, sounds, heat, cold, light, colors, and the like. These sensations, in truth, represent nothing that exists outside our mind and vary according to the different ways the body is affected.
At the same time, the mind also perceived magnitudes, shapes, and motions — but these were presented not as sensations, but as things or properties of things that exist, or at least could exist, outside the mind. However, at that age the mind did not yet notice this difference between the two kinds of perception.
Later, when the body — which nature has designed to move itself in various directions — began pursuing what was beneficial and avoiding what was harmful, the mind, closely connected to it, started to notice that the things pursued or avoided existed outside itself. The mind then attributed to these external objects not only magnitudes, shapes, and motions — which it grasped as real properties of things — but also tastes, smells, and other sensations that were actually produced within the mind itself. And since the mind only considered other objects insofar as they were useful to the body it was joined to, it judged that things had more or less reality depending on how strongly they affected the body. This is how people came to believe there was more substance in rocks and metals than in air or water — because they felt more hardness and weight in them. Air was thought to be practically nothing, as long as it was neither moving, nor hot, nor cold. The stars appeared to be no bigger than candle flames, since that was all the light they seemed to give off. The mind did not notice that the earth rotates on its axis or that its surface is curved like a globe, and so it readily judged the earth to be flat and motionless. Our minds have been filled since infancy with a thousand other prejudices of this sort, which in our youth we forgot we had accepted without proper examination, treating them as if they were the highest certainty — as if they were known through our senses or implanted in us by nature.
Now, in our mature years, when the mind is no longer entirely subject to the body and does not refer everything to it — and instead also seeks to discover the truth about things as they are in themselves — we do recognize that many of the judgments we formed in childhood were false. But we find it very difficult to erase them from our memory, and as long as they remain there, they give rise to various errors. For example, since from our earliest years we imagined the stars to be very small, we find it extremely hard to shake that impression, even when astronomical reasoning plainly tells us they are enormously large. So powerful is the grip of preconceived opinion.
Beyond this, our mind cannot focus on any object for long without eventually feeling some strain and fatigue. And of all objects, it has the greatest difficulty focusing on those that are present neither to the senses nor to the imagination — whether because this is natural to it given its union with the body, or because in our early years, being occupied only with perceptions and images, it became more practiced in those modes of thinking than in any other. This is also why many people are unable to conceive of any substance except what is physical and even perceptible to the senses. They are unaware that the only objects of imagination are those that involve extension, shape, and motion, while there are many other things that are purely intelligible. They convince themselves that nothing can exist except body, and that no body exists that cannot be sensed. And since in truth we never perceive any object exactly as it is through the senses alone — but only through our reason applied to what the senses give us, as will be shown later — the result is that most people throughout their lives perceive nothing except in a confused way.
Finally, because we attach all our ideas to words in order to express them and commit our thoughts to memory in connection with those words, and because we later find it easier to recall the words than the things they stand for, we can scarcely conceive of anything so distinctly that we completely separate what we conceive from the words chosen to express it. As a result, most people pay more attention to words than to things. They frequently assent to terms without attaching any meaning to them at all, either because they think they once understood the terms, or because they assume they learned them from others who understood them correctly. This is not the place to discuss this matter in detail, since the nature of the human body has not yet been explained, nor has the very existence of body been established. But enough has been said to enable us to distinguish our clear and distinct conceptions from our obscure and confused ones.
If we want to philosophize seriously and devote ourselves to discovering all the truths we are capable of knowing, we must, first of all, set aside our prejudices. In other words, we must take great care to withhold our assent from the opinions we have previously accepted until, upon fresh examination, we find them to be true. Next, we must make an orderly review of the ideas in our minds and accept as true all and only those that we clearly and distinctly perceive.
By doing this, we will first discover that we exist insofar as it is our nature to think, and at the same time that there is a God on whom we depend. After considering his attributes, we will be able to investigate the truth of all other things, since God is their cause. Besides our knowledge of God and of our own mind, we will also find that we possess knowledge of many eternally true propositions — for example, that nothing cannot be the cause of anything. We will further discover in our minds the knowledge of an extended, physical nature that can be moved, divided, and so on, as well as certain sensations that affect us — such as pain, colors, and tastes — even though we do not yet know why we are affected in these ways.
Comparing what we have now learned, through this orderly examination, with our earlier confused understanding, we will develop the habit of forming clear and distinct ideas of everything we are capable of knowing. In these few principles, I believe the most general and important foundations of human knowledge are contained.
Above all, we must impress this infallible rule upon our memory: what God has revealed is incomparably more certain than anything else, and we should submit our belief to divine authority rather than to our own judgment — even if the light of reason should seem, with the greatest clarity and evidence, to suggest something contrary to what is revealed. But in matters on which there is no revelation, it is entirely inconsistent with the character of a philosopher to accept as true what has not been verified, or to trust the senses — that is, the thoughtless judgments of childhood — more than the considered conclusions of mature reason.
Although we are all fairly convinced that material things exist, we previously called this into question and counted the belief in their existence among the prejudices of childhood. So now we need to investigate the grounds on which we can know this truth with certainty.
First, there is no doubt that every perception we have comes to us from some object different from our mind. After all, it is not within our power to choose which perception we experience rather than another — our perceptions depend entirely on whatever object is affecting our senses. We might still ask whether that object is God or something different from God. But because we perceive — or rather, because our senses prompt us to clearly and distinctly grasp — a certain matter that is extended in length, breadth, and thickness, whose various parts have different shapes and motions, and which gives rise to our sensations of color, smell, pain, and so on, God would without question deserve to be called a deceiver if he directly presented this idea of extended matter to our minds himself, or caused it to be presented by some object that had no extension, shape, or motion.
For we clearly conceive of this matter as entirely distinct from God and from ourselves (that is, from our minds). We even seem to clearly recognize that our idea of matter is formed on the occasion of objects existing outside our minds — objects to which the idea is in every way similar.
But since God cannot deceive us — for deception is contradictory to his nature, as we have already established — we must conclude without hesitation that there really does exist a certain object extended in length, breadth, and thickness, possessing all the properties we clearly grasp as belonging to extended things. This extended substance is what we call body or matter.
We should also conclude that a certain body is more closely united to our mind than any other. We can see this because pain and other sensations affect us without any warning, and the mind recognizes that these sensations do not come from itself alone. They do not belong to the mind insofar as it is a thinking thing, but only insofar as it is united to another thing — something extended and movable — which is called the human body. But this is not the place to discuss that matter in detail.
It will be enough to note that the perceptions of the senses should be understood as relating to this intimate union of the human body and mind. Our senses usually make us aware of what, in external objects, may be useful or harmful to this union, but they do not show us those objects as they really are in themselves — except perhaps occasionally and by accident.
Once we have taken this observation on board, we can easily set aside the prejudices that come from relying on the senses. Instead, we can turn to our understanding alone by carefully reflecting on the ideas that nature has implanted in it.
In this way, we will discover that the nature of matter or body, considered in general, does not consist in its being hard, heavy, colored, or in any other quality that affects our senses. It consists simply in its being a substance extended in length, breadth, and depth.
Consider hardness: all we actually know about it from the senses is that the parts of hard bodies resist the motion of our hands when we touch them. But if every time we reached toward something, all the bodies in that spot moved away as quickly as our hands approached, we would never feel any hardness — and yet we would have no reason to think those bodies had lost what makes them bodies. So the nature of body does not consist in hardness.
The same can be shown for weight, color, and all other qualities of this kind that we perceive in physical matter. We can strip them all away, and the body still remains a body. It follows that the nature of body depends on none of these properties.
Two issues still stand in the way of fully accepting that the true nature of body consists in extension alone.
The first is the widespread belief that most bodies can be rarefied and condensed — that is, when rarefied they have more extension than when condensed. Some thinkers have even gone so far as to distinguish between the substance of a body and its quantity, and between quantity itself and extension.
The second issue is this: where we conceive only of extension in length, breadth, and depth, we do not normally say that a body is there. We say only that there is space, or even empty space — and most people believe this "empty space" is a mere nothing.
Regarding rarefaction and condensation: anyone who pays careful attention to their own thinking, and accepts only what they are clearly aware of, will realize that these processes involve nothing more than a change of shape in the body being rarefied or condensed.
In other words, rare (low-density) bodies are those that have many gaps between their parts, and these gaps are filled by other bodies. Dense bodies, on the other hand, are those whose parts have moved closer together, reducing or eliminating these gaps entirely. When the gaps are completely closed, the body becomes absolutely dense.
But here is the key point: a body, when condensed, does not actually have less extension than when its parts are spread apart over a larger space. We should not count the extension of the pores or gaps — which the body's own parts do not occupy when it is rarefied — as belonging to that body. That extension belongs to the other bodies filling those gaps. It is just like a sponge full of water: we do not think that each part of the sponge has become larger because the sponge is swollen with water. We recognize that the sponge's pores are simply wider, and the body of the sponge is therefore spread over a larger space.
And honestly, I cannot see the force of the arguments that have led some to say that rarefaction results from an increase in the body's own quantity. It is much better to explain it the way we explain a sponge.
True, when air or water is rarefied, we cannot see the pores that have enlarged, or the new body that has come in to fill them. But it is far less reasonable to invent some unintelligible explanation — just to give a verbal, merely apparent account of rarefaction — than it is to conclude that there are pores or gaps between the parts that have grown larger and been filled with some new body. We should not hesitate to accept this explanation just because we cannot perceive this new body with our senses. There is no reason to believe that we must be able to perceive every body that exists with our senses.
We can see that rarefaction is very easy to explain in this way, and impossible to explain any other way. After all, there would be an obvious contradiction in supposing that a body could gain extension that it did not have before, without the addition of new extended substance — in other words, without the addition of another body. It is impossible to conceive of added extension or quantity without the substance that has that extension or quantity, as will become even clearer from what follows.
Quantity differs from extended substance, and number from the thing numbered, not in reality but only in our thought. For example, we can think about the entire nature of a physical substance that fills a space of ten feet, without paying attention to that specific measure of ten feet. The reason is obvious: the thing we are thinking about has the same nature in any part of that space as it does in the whole. Conversely, we can think about the number ten, or about a continuous quantity of ten feet, without thinking about any specific substance. The concept of the number ten is clearly the same whether we are counting ten feet, ten apples, or ten of anything else. And we can think about a continuous quantity of ten feet without thinking about this or that particular substance — though we cannot think about it without some extended substance of which it is the quantity.
In reality, however, it is impossible to take away even the smallest part of such quantity or extension without also removing an equal amount of the substance. And conversely, we cannot reduce the substance without also reducing the quantity or extension by the same amount.
Some people may express themselves differently on this point, but I am nevertheless convinced they do not actually think differently from what I have just said. When they distinguish physical substance from extension or quantity, they either mean nothing at all by the word "substance," or they are forming a confused idea of non-physical substance in their minds and mistakenly applying it to what is physical — while leaving to extension the true idea of physical substance. They then call this extension an "accident," but with such obvious inconsistency that it is easy to see their words do not match their actual thoughts.
Space — or internal place — and the physical substance contained in it are not different in reality. They differ only in how we typically think about them.
For in truth, the very same extension in length, breadth, and depth that constitutes space also constitutes body. The only difference lies in this: when it comes to body, we think of extension as particular to that body, and we think of it as changing location when the body moves. When it comes to space, we give extension a more general, abstract character. After we remove a body from a certain space, we do not think we have removed the extension of that space along with it. The extension seems to remain, as long as the space keeps the same size and shape and maintains its position relative to the surrounding bodies by which we define it.
It is actually easy to see that the same extension which makes up the nature of body also makes up the nature of space, and that these two things differ from each other only the way a genus or species differs from an individual. Consider this: take the idea we have of any body — a stone, for instance — and strip away everything that is not essential to its being a body.
First, hardness can go: if the stone were melted or ground to powder, it would lose its hardness but would not stop being a body. Color can go too: we have seen stones so transparent they are essentially colorless. Weight can go: fire is extremely light, yet it is still a body. And finally, we can remove cold, heat, and every other quality of this kind — either because these are not considered part of the stone itself, or because changing them does not cause the stone to stop being a body.
After this process of elimination, we find that nothing remains in the idea of body except that it is something extended in length, breadth, and depth. And this is exactly what is contained in our idea of space — not just space that is filled with body, but even what is called empty space.
There is, however, a difference in how we think about them. If we remove a stone from the space it occupied, we think of its extension as being removed along with it, since we regard the extension as particular to and inseparable from the stone. But at the same time, we suppose that the extension of the place where the stone was still remains — even though that place is now occupied by wood, water, air, or some other body, or even if it is supposed to be empty. We think this because we are now considering extension in general, and we believe the same extension can be common to stones, wood, water, air, and other bodies — and even to a vacuum, if there is such a thing — provided the space keeps the same size and shape and maintains the same position relative to the external bodies that define it.
The reason for this is that the words "place" and "space" signify nothing really different from the body that is said to be in a place. They merely designate that body's size, shape, and position relative to other bodies. To determine a thing's position, we need to refer to certain other bodies that we treat as stationary. Depending on which bodies we choose as our reference points, we can say that the same thing both does and does not change its place at the same time.
For example, when a ship is sailing out to sea, a person sitting at the stern can be said to stay in one place — if we are looking at the parts of the ship, since he maintains the same position relative to them. But if we look to the neighboring shores, the same person seems to be constantly changing place, since he is steadily moving away from one shore and toward another. And further, if we suppose that the earth itself is rotating, carrying everything from west to east at the same rate that the ship moves from east to west, we will again say the person at the stern has not changed his place — because his place will be determined by certain points that we imagine to be fixed in the heavens.
But if we finally become convinced that there are no truly fixed points anywhere in the universe — as will later be shown to be probable — we will conclude from this that nothing has a permanent place except insofar as we fix it in our minds.
The terms "place" and "space" do differ in meaning, however. "Place" emphasizes position more than size or shape, while "space" makes us think primarily of size and shape. We often say that one thing has taken the place of another, even though it does not have exactly the same size or shape. But we would not say it occupies the same space unless it does. When something's position changes, we say its place has changed, even if the size and shape remain the same. So when we say that a thing is in a particular place, we mean only that it is situated in a certain way relative to other objects. And when we add that it occupies a certain space, we mean in addition that it has a specific size and shape that exactly fill that space.
We never distinguish space from extension in length, breadth, and depth. Sometimes, however, we think of place as being in the thing placed, and other times as being outside it. Internal place is no different from space. But external place can be understood as the surface that immediately surrounds the thing placed.
It should be noted that by "surface" here we do not mean any part of the surrounding body. We mean only the boundary between the surrounding body and the surrounded body — which is nothing more than a mode, not a thing in itself. Or at least, we are speaking of surface in general, which is not part of one body more than another, and which is always considered the same so long as it retains the same size and shape.
Even if the entire surrounding body were replaced, along with its surface, we would not say that the body inside it had changed its place — as long as it maintained the same position relative to the other bodies we treat as stationary. For example, if a boat is being carried in one direction by the current of a river, and pushed in the opposite direction by the wind with equal force, so that its position relative to the banks does not change, we would readily say it remains in the same place — even though the water surrounding it is constantly being replaced.
As for a vacuum in the philosophical sense of the word — that is, a space in which there is absolutely no substance — it is clear that no such thing exists. The extension of space (or internal place) is not different from the extension of body. Since a body has extension in length, breadth, and depth, and that is our reason for concluding it is a substance, it would be flatly contradictory to say that nothing can have extension. So we must draw the same conclusion about supposedly empty space: since there is extension in it, there must also be substance in it.
In everyday usage, when we use the word "vacuum," we do not actually mean a place or space in which there is absolutely nothing. We mean only a place where there is none of the things we expect to find there.
A pitcher made to hold water is called "empty" when it is merely full of air. A fishpond is said to have "nothing" in it when it has no fish, even though it is full of water. A cargo ship is said to be "empty" when, instead of the merchandise it was designed to carry, it is loaded only with sand as ballast. And in the same way, we call a space "empty" when it contains nothing we can detect with our senses, even though it may contain real, self-existing matter.
For we do not normally pay attention to the bodies around us unless they produce impressions on our sense organs that are strong enough for us to perceive them. If, forgetting what these words properly mean, we go on to suppose that a space we called "empty" contains not just no detectable object, but no object at all, we will be making the same mistake as if we decided that the air inside a pitcher is not a real substance just because, in everyday speech, the pitcher is said to be empty.
Nearly all of us have fallen into this error from our earliest years. Observing that there is no necessary connection between a container and the particular body it holds, we assumed that God could at least remove a body from a container without anything else having to take its place.
To correct this false belief, we need to recognize that while there is no necessary connection between a container and the particular body inside it, there is an absolutely necessary connection between the concave shape of the container and the extension that must fill that cavity. It is no more contradictory to conceive of a mountain without a valley than to conceive of such a cavity without the extension it contains — or that extension apart from an extended substance. For, as we have repeatedly said, nothing cannot have extension.
So if someone asks what would happen if God removed all the body from a container without allowing any other body to take its place, the answer must be this: the sides of the container would come together. For when two bodies have nothing between them, they must be touching each other. It is plainly contradictory for two bodies to be separated — for there to be a distance between them — and yet for that distance to be nothing at all. Every distance is a mode of extension, and therefore cannot exist without an extended substance.
Now that we have established that the nature of physical substance consists only in its being an extended thing, and that this extension is no different from the extension we attribute to space (however empty it seems), several things follow easily.
First, it is impossible for any part of matter to occupy more space at one time than at another — and so rarefaction can only work in the way explained above (through the filling of pores by other bodies).
Second, there cannot be more matter in a container when it is filled with lead, gold, or any other body — however heavy or hard — than when it contains only air and is thought to be empty. The quantity of matter a body contains does not depend on its weight or hardness, but only on its extension, which is always the same for the same container.
We also discover from this that there cannot exist any atoms — that is, parts of matter that are by their very nature indivisible. No matter how small we suppose these parts to be, since they must still be extended, we can always divide any one of them in thought into two or more smaller parts. And so we must acknowledge that they are divisible. For there is nothing we can divide in thought that we do not thereby recognize to be genuinely divisible. If we were to judge something indivisible, our judgment would contradict what we know about it.
Even if we were to suppose that God had reduced a particle of matter to such extreme smallness that no creature could divide it further, we still could not properly call it indivisible. Although God may have made it so that no created being has the power to divide it, he could not have deprived himself of the ability to do so, since it is absolutely impossible for God to diminish his own omnipotence, as we noted before. Therefore, strictly speaking, even the smallest extended particle is always divisible, because that is its very nature.
We further discover that this world — the whole universe of physical substance — is extended without limit. Wherever we try to set a boundary, we can still imagine spaces beyond it that are indefinitely extended, and we perceive that these spaces are truly as we imagine them — that is, they really do exist. They therefore contain physical substance that extends indefinitely, since, as we have already shown at length, the idea of extension that we conceive in any space is exactly the same as the idea of physical substance.
It also follows easily from all this that the earth and the heavens are made of the same matter. Even if there were infinitely many worlds, they would all be made of this same matter. A plurality of worlds is therefore impossible, because we clearly understand that the matter whose nature consists solely in being an extended substance already completely fills all the imaginable spaces where these other worlds could be. We cannot find in ourselves the idea of any other kind of matter.
There is therefore only one kind of matter in the entire universe, and we know it solely by its being extended. All the properties we can clearly identify as belonging to it come down to one thing: its capacity to be divided and moved in its parts. Accordingly, matter is capable of every change that can arise from the motion of its parts. Merely dividing matter in thought does not change it in any way. All real variation in matter — all diversity of form — depends on motion.
Even the ancient philosophers seem to have recognized this universally. They said that nature was the principle of motion and rest, and by "nature" they meant whatever it is that makes physical things behave the way we observe them to behave.
Motion — and here I mean local motion, because I cannot conceive of any other kind, and so I do not think we should suppose any other kind exists in nature — in the ordinary sense of the word, is nothing more than the action by which a body passes from one place to another.
Just as we noted above that the same thing can be said to both change and not change its place at the same time, we can also say that the same thing is both moving and not moving simultaneously. A person sitting in a ship that is setting sail, for example, thinks he is moving if he looks at the shore he has left and considers it fixed. But he does not think he is moving if he looks at the ship itself, among whose parts he always maintains the same position.
Moreover, because we are used to thinking that motion requires action and that rest is the absence of action, the seated person is more properly said to be at rest than in motion, since he is not conscious of performing any action.
But if, instead of relying on ordinary usage, which has no real foundation, we want to understand what motion really is, we can define it more precisely as follows: motion is the transfer of one part of matter, or one body, from the vicinity of those bodies that are in immediate contact with it and that we regard as at rest, to the vicinity of other bodies.
By "one body" or "one part of matter," I mean everything that is transferred together as a unit, even if it may be made up of many parts that have their own separate motions. And I say it is the transfer itself, not the force or action that causes the transfer. I make this distinction to show that motion is always in the thing that moves, not in the thing that causes the movement — a distinction we are not usually careful enough to make.
Furthermore, I understand motion to be a mode of the moving thing, not a substance in its own right — just as shape is a property of the thing that has a shape, and rest is a property of the thing at rest.
Now that we have established certain principles about the physical world — principles arrived at not through the biases of the senses but through the light of reason, and possessing such strong evidence that we cannot doubt their truth — the question is whether we can use these principles alone to explain everything we observe in nature. We will start with the most general phenomena, the ones on which everything else depends: the overall structure of this entire visible universe.
But before we can properly investigate this, two things must be kept in mind.
The first is that we should always remember the infinite power and goodness of God. We should not worry about making the mistake of imagining his works to be too grand, too beautiful, or too perfect. Instead, we should be careful not to assume limits to his creation when we have no certain knowledge of such limits — for that would be to think less of God's power than we should.
The second is that we should be careful not to think too highly of ourselves. We would be guilty of this if we claimed to know the boundaries of the universe without being certain of them through either natural reasoning or divine revelation — as though the reach of our thought extended beyond what God has actually made. And we would be even more guilty if we convinced ourselves that God created everything solely for our benefit, or if we imagined that our intellect could fully grasp the purposes God had in mind when creating the universe.
Now, as far as morality goes, it may be a perfectly good and pious idea to believe that God made everything for us, since this belief can inspire greater gratitude and love toward him. And in a certain sense, it is even true — there is nothing in creation that we cannot put to some use, even if only as an exercise for our minds in studying it and honoring God because of it. But it is by no means likely that everything was created for us in the sense that God had no other purpose in making it. In fact, this assumption would be plainly absurd and foolish when it comes to understanding the physical world. After all, we have no doubt that many things exist — or once existed and have since ceased to be — that were never seen or known by any human being, and were never of any use to us at all.
I would add nothing further to this Fourth Part of the Principles of Philosophy if I were carrying out my original plan of writing a Fifth and Sixth Part — one on living things (animals and plants) and the other on human beings. But I have not yet gained enough knowledge of everything I would want to cover in those two final parts, and I do not know whether I will ever have the time to finish them. So I will add a few things here about the objects of our senses, to avoid delaying the publication of the earlier parts for the sake of the later ones, or leaving out anything that those later parts might have explained.
Here is the issue: up to now, I have described the earth — and indeed the whole visible universe — as though it were nothing more than a machine, where the only things to consider were the shapes and motions of its parts. But the senses present us with many other things as well: colors, smells, sounds, and so on. If I said nothing about these at all, it would seem like I had skipped over most of the objects that exist in nature.
We must understand, then, that although the human soul is united with the entire body, its principal seat is in the brain. It is in the brain alone that it understands, imagines, and perceives. And it does this through the medium of the nerves, which stretch like threads from the brain to all the other parts of the body. These nerves are connected to the body's parts in such a way that we can hardly touch any spot without moving the tips of some nerves spread over it. This motion travels to the other ends of those nerves, which are gathered together in the brain around the seat of the soul, as I have already explained in detail in the fourth chapter of the Dioptrics.
The movements that the nerves transmit to the brain affect the soul or mind — which is intimately joined to the brain — in different ways, depending on the variety of the motions themselves. These different effects on the mind, the thoughts that immediately arise from these motions, are what we call perceptions of the senses, or as we commonly say, sensations.
The varieties of these sensations depend, first, on differences between the nerves themselves, and second, on differences in the movements within each nerve. We do not, however, have as many distinct senses as we have nerves. We can distinguish only seven principal classes of nerves, two of which belong to the internal senses and the other five to the external senses.
The nerves that extend to the stomach, the esophagus, the throat, and the other internal organs that serve our natural needs make up one of our internal senses. This is called natural appetite.
The other internal sense covers all the emotions of the mind — passions and feelings like joy, sadness, love, hate, and so on. This sense depends on the nerves that extend to the heart and the area around the heart, and these nerves are extremely small. To give an example: when the blood happens to be pure and well balanced, so that it expands in the heart more readily and forcefully than usual, this enlarges and stimulates the tiny nerves scattered around the heart's openings. That movement is transmitted to the brain, where it gives the mind a natural feeling of joy. And whenever these same nerves are stimulated in the same way — even if the cause is something completely different — they produce the same feeling of joy in our mind.
So the mere imagination of something good does not itself contain the feeling of joy. What happens instead is that it causes the animal spirits to flow from the brain to the muscles where these nerves are inserted. This dilates the openings of the heart, which in turn stimulates those tiny nerves to move in the way nature designed to produce the sensation of joy.
Here is how it works in practice: when we receive news, the mind first judges it. If the news is good, the mind rejoices with what we might call intellectual joy — a joy that is independent of any bodily reaction, and which even the Stoics would have allowed their wise man to feel (despite their claim that he should be free from all passion). But as soon as this joy passes from the understanding to the imagination, the spirits flow from the brain to the muscles around the heart. There they stimulate the tiny nerves, which in turn causes another motion in the brain. This second motion gives the mind the sensation of what we might call animal joy — a bodily, felt joy.
The same principle works in reverse. When the blood is so thick that it flows only sparingly into the heart's chambers and does not expand enough there, it stimulates those same nerves in a completely different way. This different motion, communicated to the brain, gives the mind the sensation of sadness — even though the mind itself may have no idea why it feels sad. And any other cause that stimulates these nerves in the same way will also produce the same feeling of sadness.
Other movements of these same nerves produce other effects: the feelings of love, hate, fear, anger, and so on, insofar as these are merely passions of the mind — that is, insofar as they are confused thoughts that the mind does not generate on its own, but receives because it is so closely joined to the body, from which it gets these impressions. There is the widest possible difference between these passions and the distinct, clear thoughts we have about what ought to be loved, chosen, or avoided — although the two are often found together.
Natural appetites like hunger, thirst, and the rest are also sensations produced in the mind through the nerves of the stomach, throat, and other parts. They are entirely different from the deliberate will to eat, drink, or do whatever we think is good for the preservation of our body. But because this will or desire almost always accompanies them, they are called appetites.
We commonly count the external senses as five, because there are five different kinds of objects that stimulate the nerves and their organs, producing five corresponding kinds of confused thoughts in the soul.
First, touch. The nerves terminating in the skin of the whole body can be stimulated through this medium by any physical objects at all. These objects move the nerves in different ways: one way by their hardness, another by their weight, another by their heat, another by their moisture, and so on. To the extent that they are moved — or blocked from their normal motion — in different ways, different sensations are produced in the mind, and we give names to a corresponding number of tactile qualities.
Beyond this, when these nerves are stimulated a bit more powerfully than usual — but not so much as to actually hurt the body — we feel a sensation of tickling, which is naturally pleasant to the mind. It is pleasant because it signals that the body the mind is joined to is strong enough to withstand the stimulus without being harmed. But if the stimulus is strong enough to actually damage the body in some way, it gives the mind the sensation of pain. This is why physical pleasure and pain, though they feel completely opposite, actually arise from nearly the same kinds of causes.
Second, taste. Other nerves, scattered over the tongue and the nearby areas, are stimulated in different ways by particles of food that separate from each other and float in the saliva in the mouth. These produce sensations of different tastes depending on the different shapes of those particles.
Third, smell. Two nerves — or rather extensions of the brain, since they do not go beyond the skull — are stimulated by particles of earthly substances that separate and float in the air. But not just any particles: only those subtle and penetrating enough to enter the pores of the spongy bone when we breathe them in through our nostrils and reach these nerves. The different motions of these particles give rise to the sensations of different smells.
Fourth, hearing. There are two nerves within the ears, attached to three tiny bones that support each other. The first of these bones rests against the small membrane that covers the cavity we call the eardrum. All the various vibrations that the surrounding air transmits to this membrane are passed along by these nerves to the mind, and these vibrations give rise to the sensations of different sounds, depending on their character.
Fifth and finally, sight. The ends of the optic nerves form the coating in the eyes called the retina. These nerves are not stimulated by air or by any earthly object, but only by the particles of what Descartes calls the "second element" — from which we get our sense of light and colors. I have already explained this at length in the Dioptrics and the treatise on Meteors.
It is clearly established, however, that the soul does not perceive through whatever part of the body it happens to be in, but only through the brain, where the nerves convey to it — through their movements — the various actions of external objects that touch the parts of the body where those nerves are located.
Here are three proofs of this.
First, there are various diseases that affect the brain alone, yet they disrupt or completely destroy our ability to sense. Sleep, for example, affects only the brain, yet every day it takes away our power of perception for a large part of our time — and restores it when we wake.
Second, even when there is no disease in the brain or in the organs of the external senses, it is enough to block the signal in one of the nerves at any point between the brain and the body part where it ends, and sensation in that body part will be lost.
Third — and most strikingly — we sometimes feel pain as though it is in a particular limb, when the actual cause is not in that limb at all, but somewhere closer to the brain along the path of the nerves. I could prove this with countless examples, but I will mention just one.
A girl suffering from a severe ulcer on her hand had her eyes bandaged whenever the surgeon came to treat it, because she could not bear to watch the wound being dressed. After the gangrene spread, her arm was amputated at the elbow without her knowledge. Bandages wrapped one above the other were placed where the amputated part had been, so that for some time she had no idea the operation had been performed. Yet she continued to complain of pains — sometimes in one finger of the hand that had been cut off, sometimes in another. The only explanation for this is that the nerves which had once stretched from the brain down to the hand, and which now ended in the arm near the elbow, were being stimulated there in the same way they would have needed to be stimulated in the hand to produce the sensation of pain in this or that finger. This clearly shows that the pain of the hand is not felt by the mind because it is in the hand, but because it is in the brain.
Next, it can be shown that the mind is of such a nature that the motions of the body alone are enough to produce every kind of thought in it, without those thoughts needing to resemble the motions that cause them. This is especially true of those confused thoughts we call sensations.
Consider this: we see that words — whether spoken aloud or merely written — can stir up all kinds of thoughts and emotions in our minds. With the same paper, the same pen and ink, by merely moving the pen across the paper in a particular way, we can write letters that will fill readers' minds with thoughts of battles, storms, or furies, along with the passions of outrage and sorrow. But if the pen is moved in a slightly different way — barely different from the first — this tiny change can produce entirely different thoughts: thoughts of peace, calm, and pleasantness, accompanied by the completely opposite passions of love and joy.
Now, someone might object that writing and speech do not directly produce passions or mental images of things that differ from the letters and sounds. They simply give us knowledge of letters and sounds, and then the mind, understanding what the words mean, generates the corresponding images and emotions on its own. But what about the sensations of pain and tickling? The mere motion of a sword cutting our skin causes pain, but it does not thereby make us aware of the motion or shape of the sword. And the sensation of pain is certainly no less different from the motion that causes it — or from the motion of the body part being cut — than our sensations of color, sound, smell, or taste are from their respective causes.
From all of this we can conclude that the mind is of such a nature that the motions of certain bodies can easily produce in it every kind of sensation, just as the motion of a sword produces the sensation of pain.
Furthermore, we observe no difference between the nerves that would make us think one set transmits something different to the brain than another, or that anything at all reaches the brain besides the local motion of the nerves themselves. And we can see that local motion alone produces in us not only tickling and pain, but also the sensations of light and sound.
If someone strikes our eye hard enough that the vibration reaches the retina, we see numerous sparks of fire — even though those sparks are not actually outside our eye. When we plug our ear with a finger, we hear a humming sound whose only cause is the agitation of the trapped air. And we frequently observe that heat, hardness, weight, and other sensible qualities in objects — as well as the forms of purely material bodies like fire — are produced by the motion of other bodies, which in turn produce motions in still other bodies.
We can easily understand how the motion of one body can be caused by the motion of another, and how it can vary based on the size, shape, and arrangement of parts. But we are completely unable to understand how these same properties — size, shape, and motion — could produce something entirely different from themselves, such as the "substantial forms" and "real qualities" that many philosophers claim exist in bodies. Nor can we understand how such forms or qualities could have the power to cause motions in other bodies.
But since we know — from the nature of our own soul — that the various motions of bodies are enough to produce every sensation the soul has; since experience confirms that many of our sensations are actually caused by such motions; and since we find no evidence that anything besides these motions ever travels from the organs of the external senses to the brain — we have good reason to conclude the following: what we perceive in external objects and call light, color, smell, taste, sound, heat, cold, and other tactile qualities — or even what some call their "substantial forms" — is nothing other than the various arrangements of those objects (in terms of size, shape, and motion) that have the power to set our nerves in motion in various ways.
From all of this, we can see — through a straightforward survey — that there is no phenomenon of nature whose explanation has been left out of this treatise. Beyond what the senses perceive, there is nothing that counts as a natural phenomenon. And setting aside motion, size, shape, and the arrangement of the parts of each body (which I have already explained as they exist in physical objects), the only things we perceive from outside ourselves through the senses are light, colors, smells, tastes, sounds, and tactile qualities. I have just shown that these are nothing more — at least as far as we can know — than certain arrangements of objects, consisting in their size, shape, and motion.
I also want to point out that, although I have tried here to explain the entire nature of physical things, I have not used any principle that was not accepted and approved by Aristotle and by philosophers of every era. In this sense, my philosophy — far from being new — is actually the most ancient and universal of all. I have truly done nothing more than consider the shape, motion, and size of bodies, and examine what must follow from their interactions according to the principles of mechanics — principles confirmed by reliable, everyday experience.
No one has ever doubted that bodies move, that they come in various sizes and shapes, that their motions vary according to these differences, and that when they collide, the larger ones break into many smaller ones, changing shape in the process. We experience the truth of this not through just one sense but through several: touch, sight, and hearing. We also clearly imagine and understand it. The same cannot be said for the other things that the senses report to us — colors, sounds, and the like. Each of these affects only one of our senses and merely imprints a confused image on our imagination, giving our understanding no clear knowledge of what it actually is.
I do, however, assume that each body contains many particles too small for any of our senses to detect. This might not sit well with those who take the senses as the measure of what can be known. But we greatly shortchange human reason if we assume it cannot go beyond what the eye can see.
No one can seriously doubt that there are bodies too small to be perceived by any of our senses — you only need to think about what is being added, moment by moment, to bodies that are growing little by little, and what is being taken away from those that are shrinking the same way. A tree grows larger every day, and it is impossible to understand how it gets bigger unless we accept that some material is being added to it. But who has ever observed with the senses those tiny particles that are added to a tree in a single day of growth? Among philosophers, at least those who hold that matter is infinitely divisible ought to admit that through division, parts can become so small as to be completely undetectable.
And really, it should not surprise us that we cannot perceive very tiny bodies. The nerves that objects must stimulate in order to cause perception are not themselves very small — they are like little cords, made up of bundles of even smaller fibers. The smallest bodies simply are not capable of moving them.
Nor do I think anyone who uses their reason would disagree with this: we do better philosophy when we explain what happens in those imperceptibly small bodies by analogy with what we observe happening in the ones we can see — and in this way explain everything in nature, as I have tried to do in this treatise — than when we explain the same things by inventing some mysterious novelties that have no relation to what we actually observe. I am talking about things like "prime matter," "substantial forms," and that whole elaborate array of supposed qualities, each of which is harder to understand than everything it is meant to explain.
Now, someone might point out that Democritus also proposed tiny particles of various shapes, sizes, and motions, and said that all perceptible bodies arise from their accumulation and interaction — yet his approach has been widely rejected. To this I reply: the philosophy of Democritus was never actually rejected because he proposed the existence of bodies smaller than those we can perceive and attributed to them various sizes, shapes, and motions. No one can doubt that such bodies really exist, as we have already shown.
His philosophy was rejected for four other reasons. First, because he assumed these particles were indivisible — and on this point, I reject his view too. Second, because he imagined there was empty space (a vacuum) around them, which I have shown to be impossible. Third, because he attributed gravity to these particles as an intrinsic property, whereas I deny that gravity belongs to any body considered by itself, since it is a relational quality that depends on how bodies are situated and moving with respect to each other. And fourth, because he never explained in detail how everything arises from the mere interaction of particles. Or if he did explain a few cases, his reasoning was far from coherent or systematic enough to extend to all of nature. This, at least, is the verdict we must reach about his philosophy based on what has been passed down to us in writing. I leave it to others to judge whether my own philosophy holds together consistently and whether it can generate enough explanations. And since the consideration of shape, size, and motion was accepted not only by Democritus but also by Aristotle and everyone else — and since I reject everything Democritus assumed except for this one point, while also rejecting what the others assumed — it should be clear that my approach has no more kinship with Democritus than with any other particular school of thought.
But since I assign specific shapes, sizes, and motions to particles too small to see, as if I had observed them directly, while admitting that they are beyond the reach of the senses — someone will naturally ask how I came by this knowledge.
The answer is this: I first considered, in general, all the clear and distinct ideas about physical things that our understanding contains. I found none besides ideas of shapes, sizes, and motions, along with the rules by which these three things can vary in relation to each other — rules that are the principles of geometry and mechanics. I concluded that all the knowledge we can have of nature must necessarily be drawn from this source, because all our other ideas about perceptible things are confused and obscure, and far from helping us understand the world, they actually get in the way.
From there, starting with the simplest and best-known principles that nature has implanted in our minds, I considered the main differences that could exist between the sizes, shapes, and positions of bodies that are imperceptible only because of their smallness, and what observable effects could be produced by their various modes of interacting. Then, whenever I found similar effects in the bodies we can perceive, I judged that those effects could have been produced in the same way — especially since no other explanation could be devised.
In this work, the example of machines made by human hands was enormously helpful. I see no difference between artificial machines and natural bodies except this: the workings of machines depend mostly on instruments large enough to be seen — since they must be proportional to the hands of those who make them — and so their shapes and motions are visible. But the workings of natural bodies almost always depend on parts so minute that they escape our senses entirely.
And it is certain that all the rules of mechanics also belong to physics, of which mechanics is a branch. Everything that is artificial is also natural: it is no less natural for a clock, made with the right number of wheels, to mark the hours than for a tree that has sprung from a particular seed to produce the fruit unique to it. Accordingly, just as those who are familiar with mechanical devices can figure out, when they are told what a machine does and shown some of its parts, how the unseen parts are likely made — so too, from examining the observable effects and parts of natural bodies, I have tried to determine the character of their hidden causes and imperceptible parts.
But someone may reply that even if I have proposed causes that could produce all natural phenomena, we should not conclude from this that nature actually works this way. After all, just as the same craftsman can build two clocks that tell time equally well and look identical on the outside but have completely different internal mechanisms, so too the Supreme Maker of all things has an infinity of different methods at his disposal. He could have made everything in the world appear exactly as it does by any one of them, and it may be impossible for the human mind to know which method he actually chose.
I freely grant this. And I believe I have accomplished everything that was required, so long as the causes I have proposed produce effects that accurately match all the phenomena of nature — without needing to determine whether these are the actual causes or whether nature uses different ones. This will be sufficient for practical purposes. Medicine, mechanics, and all the other arts that depend on knowledge of physics aim only at effects that are observable — effects that count as phenomena of nature.
And in case anyone thinks Aristotle achieved or claimed anything more than this, it should be remembered that he himself expressly says, at the beginning of the seventh chapter of the first book of his Meteorology, that when dealing with things that are not directly observable, he considers it sufficient to show that they could be as he describes them.
Nevertheless, so as not to sell the truth short by treating it as less certain than it really is, I want to distinguish between two kinds of certainty.
The first kind is called moral certainty — that is, certainty sufficient for the conduct of everyday life, even though it might turn out to be false if we consider the absolute power of God. For example, people who have never been to Rome do not doubt that it is a city in Italy, even though it is theoretically possible that everyone who told them so was wrong.
Here is another analogy. Suppose someone wants to decipher a letter written in Latin characters that have been scrambled, and they try reading a B wherever they find an A, a C wherever they find a B, and so on, substituting each letter with the next one in the alphabet. If this method produces recognizable Latin words, they will not doubt that they have found the letter's true meaning, even though they discovered it only by guessing, and even though the writer might have used a different code and hidden a different message. This is so improbable — especially when the decoded text contains many words — that it seems almost impossible.
Now, those who observe how many things about magnets, fire, and the entire structure of the world have been deduced in this treatise from just a handful of principles — even if they initially thought I had chosen those principles randomly and without basis — may come to acknowledge that it would be extremely unlikely for so many results to fit together if those principles were false.
Beyond this, there are some things — even in the natural world — that we can judge to be absolutely certain. Absolute certainty arises when we judge that it is impossible for something to be different from how we think it is. This certainty rests on a metaphysical foundation: God is supremely good and the source of all truth, and so the faculty he gave us for distinguishing truth from error cannot deceive us, as long as we use it properly and perceive something clearly and distinctly through it.
Truths of this kind include mathematical proofs, the knowledge that physical things exist, and all the clear reasoning we construct about them. The results I have presented in this treatise may perhaps be admitted to this class of absolute certainties, if one considers that they are deduced in an unbroken chain from the very first and most basic principles of human knowledge. This is especially true once we accept that we cannot perceive external objects unless some local motion occurs in our nerves, and that such motion cannot be caused by the distant fixed stars unless motion also takes place in them and in all the heavens between them and us. Once these points are granted, everything else — at least the more general claims I have made about the world and the earth — will appear to be virtually the only possible explanations for the phenomena they address.
Nevertheless, I do not want to presume too much. I assert nothing as final, but submit all these opinions of mine to the authority of the church and the judgment of those wiser than myself. And I ask no one to believe anything I have said unless they are compelled to accept it by the force and evidence of reason.
POSTULATES
First, I ask my readers to consider how weak the reasons are that have led them up to now to trust their senses, and how uncertain all the judgments are that they have built on that trust. I ask them to turn this thought over in their minds long enough and often enough that they eventually develop the habit of no longer placing such confident faith in the senses. I believe this is necessary for anyone who wants to be capable of grasping metaphysical truths.
Second, I ask them to reflect on their own mind — on all those attributes of the mind that they will find they cannot doubt, even if they were to suppose that everything they have ever received from the senses is entirely false. They should keep reflecting on this until they have developed the habit of conceiving the mind clearly, and of believing that it is easier to know than any physical object.
Third, I ask them to carefully examine those self-evident propositions that they will find within themselves, such as: the same thing cannot both be and not be at the same time; nothing cannot be the productive cause of anything; and others like these. In this way, they should exercise the natural clarity of understanding they have been given — a clarity that the perceptions of the senses tend to disturb and obscure. They should exercise it, I say, in its pure state, freed from the objects of sense. If they do, the truth of the following axioms will become very clear to them.
Fourth, I ask them to examine the ideas of those natures that contain an assemblage of several attributes, such as the nature of a triangle, the nature of a square or other geometric figure, the nature of mind, the nature of body, and above all the nature of God — that is, of a supremely perfect being. They should observe that we can truthfully affirm of these things everything that we clearly see to be contained in them. For example, since the nature of a right-angled triangle contains this property — that its three angles are equal to two right angles — and since the nature of body (or extended thing) contains divisibility — for we cannot conceive of any extended thing so small that we could not divide it, at least in thought — it is true that the three angles of a right-angled triangle are equal to two right angles, and that all body is divisible.
Fifth, I ask them to spend significant time contemplating the supremely perfect Being. Among other things, they should consider that the ideas of all other natures contain merely possible existence, but the idea of God contains not just possible but absolutely necessary existence. From this alone, without any further reasoning, they will discover that God exists. And this will be no less self-evident than the fact that two is an even number and three is an odd one, or any other truth of this kind. For there are certain truths that are immediately obvious to some people without proof, though others may need a chain of reasoning to grasp them.
Sixth, I ask them to carefully consider all the examples of clear and distinct perception, and all the examples of obscure and confused perception, that I discussed in my Meditations. In this way, they should train themselves to distinguish what is clearly known from what is obscure. This skill is better learned through examples than through rules, and I believe I have opened up — or at least touched on — all the relevant examples in that work.
Seventh, I ask my readers, noting that they have never discovered any falsehood in things they clearly perceived, and that they have never found truth (except by accident) in things they perceived only obscurely, to consider how utterly irrational it would be — merely because of certain prejudices of the senses or some hypothesis involving unknown elements — to doubt what is clearly and distinctly perceived by the pure understanding. If they do this, they will readily accept the following axioms as true and beyond doubt. I will admit that several of these axioms could have been explained more fully, and that they ought perhaps to have been presented as theorems rather than axioms, had I wanted to be more precise.
AXIOMS
For example, how do we know that the sky exists? Because we see it? But this act of seeing does not affect the mind except insofar as it is an idea — an idea residing in the mind itself, not an image painted on the imagination. And based on this idea, we can only judge that the sky exists if we assume that every idea must have a cause of its objective reality that actually exists. We judge that cause to be the sky itself. And the same reasoning applies in every other case.
PROPOSITION I The existence of God is known from the consideration of his nature alone.
DEMONSTRATION:
To say that an attribute is contained in the nature or concept of a thing is the same as saying that this attribute is true of the thing and can be affirmed of it. (Definition IX)
But necessary existence is contained in the nature or concept of God. (Axiom X)
Therefore, it may truthfully be said that necessary existence is in God — that is, that God exists.
This syllogism is the same one I used in my reply to the sixth set of objections. Its conclusion can be known without proof by those who are free from prejudice, as I noted in Postulate V. But since it is not easy for everyone to achieve such clarity of mind, we will try to establish the same conclusion by other means.
PROPOSITION II The existence of God is demonstrated, from the fact alone that his idea is in us.
DEMONSTRATION:
The objective reality of each of our ideas requires a cause in which that same reality is contained, not merely objectively, but formally or eminently. (Axiom V)
But we have in us the idea of God (Definitions II and VIII), and the objective reality of this idea is not contained in us either formally or eminently (Axiom VI), nor can it be contained in anything other than God himself. (Definition VIII)
Therefore, this idea of God that is in us requires God as its cause. Consequently, God exists. (Axiom III)
PROPOSITION III The existence of God is also demonstrated from the fact that we ourselves, who possess the idea of him, exist.
DEMONSTRATION:
If I possessed the power to keep myself in existence, I would also have the power — even more easily — to give myself all the perfections I lack (Axioms VIII and IX), since those perfections are merely attributes of substance, and I myself am a substance.
But I do not have the power to give myself these perfections; otherwise I would already have them. (Axiom VII)
Therefore, I do not have the power to keep myself in existence.
Furthermore, I cannot exist without being preserved in existence for as long as I exist — either by myself (if I had the power) or by some other being who does have this power. (Axioms I and II)
But I do exist, and yet I do not have the power to preserve myself, as I have just shown. Therefore, I am being preserved in existence by something else.
Furthermore, whatever preserves me in existence contains in itself, formally or eminently, everything that is in me. (Axiom IV)
But I have in me the awareness of many perfections that I lack, as well as the idea of God. (Definitions II and VIII) Therefore, the awareness of those same perfections is in the being that preserves me.
Finally, this same being that preserves me cannot have an awareness of any perfections that it lacks — that is, perfections that it does not have in itself, formally or eminently. (Axiom VII) For since it has the power to preserve me, as I just said, it would certainly have the power — even more easily — to give itself those perfections if it lacked them. (Axioms VIII and IX)
But it has the awareness of all the perfections that I recognize as lacking in myself, and that I conceive can exist only in God, as I have just proved.
Therefore, it has all these perfections in itself, formally or eminently. And so it is God.
COROLLARY God has created the heavens and the earth and everything in them. Moreover, he can make all the things that we clearly conceive, in exactly the manner in which we conceive them.
DEMONSTRATION:
All of this follows clearly from the preceding proposition. For in it we proved that God exists, from the fact that there must be some being in whom are contained, formally or eminently, all the perfections of which we have any idea.
But we have in us the idea of a power so great that the being who possesses it alone must have created the heavens and the earth, and must also be capable of producing everything else we can conceive as possible.
Therefore, in proving God's existence, we have also proved all of this along with it.
PROPOSITION IV The mind and the body are really distinct.
DEMONSTRATION:
Everything that we clearly conceive can be made by God in the manner in which we conceive it. (By the preceding Corollary)
But we clearly conceive mind — that is, a thinking substance — without body, meaning without any extended substance. (Postulate II) And on the other hand, we just as clearly conceive body without mind (as everyone admits).
Therefore, at least through the omnipotence of God, the mind can exist without the body, and the body without the mind.
Now, substances that can exist independently of each other are really distinct. (Definition X)
But the mind and the body are substances (Definitions V, VI, and VII) that can exist independently of each other, as I have just shown.
Therefore, the mind and the body are really distinct.
I should note that I have invoked the omnipotence of God here as the basis for my proof — not because some extraordinary power is needed to separate the mind from the body, but simply because, in the preceding propositions, I dealt only with God, and so I had no other foundation available. It matters very little by what power two things are separated; what matters is that we can discover they are genuinely distinct.
End of Philosophical Works