Descartes

Chapter 1

The man who doubted everything

"I remained the whole day shut up alone in a stove-heated room, where I had complete leisure to occupy myself with my own thoughts."

He was born René Descartes on 31 March 1596, in a small town in the Touraine, in central France, between Tours and Châtellerault. The town is now simply called Descartes; it was renamed in his honor, first in 1802 and finally in 1967. (The Latin form of his name, Renatus Cartesius, is why his philosophy is called "Cartesian" and his coordinate grid the "Cartesian plane.") His mother died when he was barely a year old, and he was a sickly child. He went at about eleven to the Jesuit Collège de La Flèche, among the best schools in Europe, where he was taught the humanist and Aristotelian-Scholastic curriculum at its finest, and where, by his own later account, he formed two convictions that would drive his whole life. The first was admiration: the mathematics he learned there was genuinely solid, certain in a way nothing else was, and he never lost his taste for it. The second was dissatisfaction: for all the centuries of accumulated learning, the deepest questions remained exactly as unsettled as ever, with the learned still disagreeing about everything that mattered. He came out of the best education on the continent convinced that none of it rested on anything firm.

A famous La Flèche detail belongs here, because it explains the strangest fact of his death. Descartes was a lifelong late riser. He had been a frail boy, and his Jesuit tutors are said to have indulged him, letting him stay in bed in the mornings, and he came to do his best thinking there, lying still in the warm dark, before the world started. He kept the habit for fifty years, and it kills him at the end of this chapter.

He took a law degree at Poitiers in 1616 and never practiced a day of law. Instead, at around twenty-two, he did something a young gentleman of means could do: he became a soldier, not a paid professional, but a gentleman volunteer, attaching himself to an army for the experience and, mostly, for the leisure it gave him to think. He served in at least two armies (the Dutch forces of Prince Maurice of Nassau, then the army of Maximilian of Bavaria in the Holy Roman Empire), and he was far more observer than combatant. In the Dutch service he met a mathematician named Isaac Beeckman, who turned his attention toward mathematical natural philosophy and, by Descartes' own correspondence, woke something up in him. It was in the second posting, in winter quarters in 1619, that the stove-heated room comes.

That winter night is actually two different events that get welded into one, and they sit at two completely different levels of reliability. The first event is the stove-heated room, and our source for it is Descartes himself: in the Discourse on the Method of 1637 he writes that he "remained the whole day shut up alone in a stove-heated room," with complete leisure for his own thoughts, and that there he conceived the first rules of his method. It comes to us as a story he is telling, eighteen years later, building the founding narrative of his intellectual life much as Augustine staged his own conversion in the Confessions (the faith-and-reason chapter). The heated room is in his own account. That is as solid as it gets, though even here, the precise date of 1619 comes from later tradition, and the famous bit about its being November rests on the second event, which is shakier.

The second event is the three dreams, and here the ground is much softer. Tradition holds that on the night of 10–11 November 1619, in that same period, Descartes had three vivid, revelatory dreams in which he believed the foundations of a new universal science were being revealed to him. He did write the dreams down, in a private notebook called the Olympica. But the Olympica is lost. It vanished; the philosopher Leibniz saw and copied parts of it before it disappeared, but the only real surviving account of the dreams comes from a biography of Descartes written by Adrien Baillet in 1691, seventy-two years after the night in question, by a writer setting out to exalt his subject. So the dreams are not Descartes' direct voice; they are the tradition Baillet recorded, working from a manuscript we can no longer check. The notebook existed and Baillet used it, so the dreams are probably broadly real, but the exact images and their meanings are Baillet's reconstruction, and a careful reader keeps the stove-room (Descartes' own word) and the dreams (Baillet's late report) at their different distances and does not fuse them into one tidy legend. The fusion is exactly the kind of thing that hardens into fake fact.

What was the project that came out of that winter, however it actually unfolded? It was breathtakingly ambitious: a single method, modeled on mathematics, that could be applied to every question worth asking and would yield certainty across the board the way geometry yields it about triangles. And he had a reason to believe mathematics could be that template, because he had personally made mathematics do something new. Around 1637 he published, as an appendix to the Discourse, a piece of work called the Géométrie that founded what we now call analytic geometry (the discovery that points on a plane can be represented as pairs of numbers, or coordinates, geometric shapes turned into algebraic equations, and problems of one kind solved with the tools of the other). That is where the grid in every math class, the Cartesian coordinate system, comes from. It matters here not as mathematics but as biography: Descartes had felt geometry yield to method like a revelation, watched certainty pour out of a few axioms by necessary steps, and he wanted to do that (start from undeniable foundations, derive everything by unbreakable links) for the whole of human knowledge. The dream of the Meditations is the dream of geometry, aimed at philosophy.

He settled, around 1628, in the Dutch Republic, and stayed for two decades. He chose it for its relative intellectual freedom and, just as much, for its anonymity (it was a busy commercial republic with little interest in the private opinions of a foreign gentleman), and he moved house constantly, by some accounts more than twenty times, guarding his solitude. The warts of the man are here too, and they belong in the story, not in a footnote. He had a daughter, Francine, born in 1635 to a servant named Helena Jans van der Strom; he acknowledged the child, and when she died of scarlet fever in 1640, at five years old, it is reportedly the one time in any surviving account that Descartes was seen openly weeping. And he was no reckless freethinker: in 1633, when he heard that the Inquisition had condemned Galileo for defending the motion of the earth, Descartes took his own completed masterwork of physics, a book called Le Monde (The World), which was also heliocentric, and suppressed it. He simply did not publish it; it came out only after his death. That was not cowardice so much as a clear-eyed reading of what it cost to do natural philosophy under the Church's authority in the seventeenth century. But it means his most comprehensive system of physics never saw print in his lifetime, and it shows that the man who told everyone to doubt all authority knew exactly which authority could not safely be doubted out loud.

The end is almost cruel in how neatly it follows from the boy who stayed in bed. In 1649 Queen Christina of Sweden summoned Descartes to Stockholm to tutor her in philosophy and help found a Swedish academy of science. The lifelong late riser, the man who did his best thinking under the blankets at mid-morning, was made to give the Queen her lessons at five o'clock in the morning, three times a week, through a Swedish winter, in what contemporaries described as a cold and drafty castle. He arrived in late 1649; by January he was ill; he died of a respiratory infection on 11 February 1650, seven weeks after the lessons began. (A fringe academic theory has since claimed he was poisoned with arsenic by a French priest worried about his influence on the Queen's Catholicism. It has been published in the scholarly literature but is not accepted by mainstream scholarship, and poison was never established; the cold, the castle, and the five a.m. summons are explanation enough.) The philosopher who had tried to make every certainty as solid as a theorem was undone by a Queen's preference for early mornings.

Chapter 2

The demolition

"I will suppose... that some Evil Spirit which is very Powerful and crafty has used all his endeavours to deceive me; I will conceive, the Heavens, Air, Earth, Colours, Figures, Sounds, and all outward things are nothing else but the delusions of Dreams."

The Meditations on First Philosophy (1641) is where the demolition runs for real, as a procedure, in cold blood, not the breezy autobiography of the Discourse. The setup is a man alone who, as he opens the first Meditation, recalls how many falsehoods he had admitted as truths in his younger years, and who has decided, once in his life, to tear down everything he believes and accept nothing back that is not certain beyond any possible doubt. And the labor-saving trick that makes the demolition feasible is this: he does not have to examine his beliefs one by one (there are too many). He only has to undermine the foundations they rest on. Knock out a foundation and everything built on it comes down together. So he goes hunting for foundations.

Level one: the senses. Almost everything he has ever believed came to him, in the end, through the senses, through sight, hearing, touch. And the senses are caught lying all the time. A straight stick looks bent where it enters water; a square tower looks round from a distance; the sun looks small. "From time to time I have found that the senses deceive," he writes, "and it is prudent never to trust completely those who have deceived us even once." So anything resting on sense-perception is, in principle, suspect. Out it goes. But some things still seem too obvious for that: surely not even perception can be doubted when the evidence is right in front of one's eyes, a man sitting here by a fire? Only a madman would doubt that. And Descartes agrees it seems unshakable, which is exactly why he needs a stronger acid.

Level two: dreams. The stronger acid. How do I know I am not asleep? When I dream, I take the dream for reality, every single time, while it is happening. I have dreamt, with total conviction, that I was sitting by a fire when in fact I was lying undressed in bed. And there is no certain mark, no test I can run right now, that proves I am awake at this moment rather than dreaming that I am awake. If I cannot rule out that I am dreaming, then "I am sitting here by the fire" might be false, and with it goes the entire external world, every particular object, my own hands, my own body. All of it could be a dream. That one move sweeps the whole material world off the table, in two sentences. But Descartes notices that something seems to survive even the dream. Whether I am awake or dreaming, two and three still make five; a square still has four sides. The general objects of mathematics seem true no matter who is sleeping. So dreams have not reached the bottom. There is a harder doubt still.

Level three: the evil genius. This is the terminal move, the strongest doubt that can be constructed, and it is one of the great thought experiments in philosophy. Dreams are not the end of it. Suppose ("I will suppose," Descartes says, building it deliberately) that there is not a good God but instead some being of enormous power and cunning, an evil genius (Molyneux's 1680 translation renders the Latin genius malignus as "Evil Spirit"; the names "evil genius" and "evil demon" come from other translators) who is bending the whole of his effort to deceiving me. This deceiver makes the sky, the earth, colors, shapes, sounds, all of it, nothing but traps laid for my belief. And he can go further than any dream: he can rig my very mind so that I go wrong even in arithmetic, so that every time I add two and three I make a mistake I cannot catch, so that the most self-evident truths I have are planted lies. Now nothing is left. Not the world, not my body, not mathematics, not the things that feel most certain of all. The senses are gone, the world is gone, the eternal truths are gone. It is total. The floor has dropped completely away.

Two things about that demon are constantly gotten wrong. First, Descartes does not believe in the evil genius. He is not announcing that a demon is probably deceiving him. The evil genius is a tool, the strongest possible challenge he can build, deliberately cranked past anything he actually fears, precisely so that whatever survives it will have survived the worst. It is hyperbolic doubt, doubt on purpose, doubt past the point of sincerity. Second, the demon is an old idea pushed to its limit, not a thing conjured from nowhere: the medieval logician Ockham had already pressed that God, being all-powerful, could do anything not outright self-contradictory, and could in principle arrange the world (or our minds) to deceive us (the faith-and-reason chapter). Descartes' deceiver is that medieval thought taken to its terminus, the ancestor of his most radical doubt. He did not invent it from nothing. He weaponized it.

The wreckage Descartes has deliberately made: everything in doubt. There is no world, no body, no mathematics, no God yet to appeal to, only a mind, alone, that has just supposed an omnipotent liar is feeding it falsehoods about literally everything. This is the bottom. It is meant to feel like the bottom. The genius of the Meditations is that it builds the demolition in stages so that what he finds next, the one thing the demon cannot reach no matter how hard he tries, arrives not as a clever slogan but as the floor stopping, the first solid thing in a free fall. The demolition was never the point. It was clearing the site.

Chapter 3

The one thing left

"Whenever this sentence I am, I exist, is spoken or thought of by Me, 'tis necessarily True."

And then the floor stops. In the second Meditation, in the middle of the total wreckage of the first, Descartes notices the one thing the evil genius cannot take, and it is almost embarrassingly simple once seen. Suppose the demon is deceiving me about absolutely everything. Fine. Then I am being deceived. And to be deceived, there has to be a me, a me for the demon to fool. The demon can make me wrong about the sky and the earth and arithmetic, but he cannot make me wrong about the fact that I, right now, am being made wrong, because there has to be a someone there to be the victim of the trick. The harder he works to convince me I am nothing, the more certain it becomes that I am something, because nothing cannot be deceived. Doubt itself proves it: doubting is a kind of thinking, and if I am thinking, something is doing the thinking, and that something is I. As Descartes lays it down, in Molyneux's words: "whenever this sentence I am, I exist, is spoken or thought of by Me, 'tis necessarily True." Every time I think it, it cannot be false. That is the bedrock. That is the one stone the fire could not burn.

Even an all-powerful liar can't deceive me if I'm not there to be deceived. The one thing that can't be faked is the fact that something is doing the thinking right now, and that something is me. Strip away the world, the body, even mathematics, and the thinker doing the stripping is still there, unstrippable. "I am, I exist." It is the floor that survives the flood. Everything else has to be rebuilt from here.

Almost every retelling of Descartes goes wrong in the same place, so the record of what the famous sentence actually is and where it actually appears needs to be precise. The world knows it as four Latin words: Cogito, ergo sum. "I think, therefore I am." And there is a near-universal belief that those words are the climax of the Meditations. They are not. They are not in the Meditations at all. What the Meditations says, at this exact moment, is the bare first-person fact just stated, "I am, I exist" (in Descartes' Latin, ego sum, ego existo), arrived at not as a tidy little syllogism but as something he cannot help affirming the instant he thinks it. There is no "therefore" in the Meditations. The slogan lives somewhere else.

The actual map, given the confusion is so total: The phrase appears, in three different books, in three different forms, at three different temperatures. (1) In the Discourse on the Method of 1637, written in French for a wide popular audience, Descartes writes "je pense, donc je suis" ("I think, therefore I am") as a discovery, a thing he says he "observed" to be true, in the flow of an autobiography. There is no Latin in his text at all; when the Latin "COGITO ERGO SUM" appears stamped into that passage in English editions, that is the translator John Veitch's own added gloss, a nineteenth-century editorial insertion, not Descartes' 1637 words. (2) In the Meditations of 1641, written in Latin for philosophers, the climax is "ego sum, ego existo" ("I am, I exist"), the result of the argument, not a tagged formula. (3) Only in the Principles of Philosophy of 1644, a systematic textbook written in Latin to replace Aristotle in the schools, does Descartes finally write the formal axiom "ego cogito, ergo sum" ("I think, therefore I am") as an explicit first principle. So the slogan everyone quotes from the Meditations is actually from the Principles; the Meditations says something subtly but importantly different; and the Discourse has the French version that the Latin tag was later glued onto. The reason this matters is not pedantry. It is that the punchy syllogism ("I think, therefore I am") makes the cogito look like a piece of logic (premise, premise, conclusion) when in the Meditations, the place it does its real philosophical work, it is not a deduction at all. It is something that cannot be doubted in the very act of doubting it. The book where the argument actually lives is the book without the slogan.

Then the question the slogan's fame buries: what does the cogito actually prove, and, just as important, what does it not? It is tempting to think Descartes has, in one stroke, recovered his whole self: his body, his history, his name, his place in the world. He has done nothing of the kind, and he is scrupulous about it. All the cogito establishes is that something that thinks exists, right now, for as long as it is thinking. It does not prove he has a body; the body was put in doubt in chapter 2 and stays in doubt. It does not prove the world is real. It does not even prove that this thinking thing persists through time when it stops thinking; strictly, it gives him this thinking, now. The certainty is real but it is thin: a single, momentary, first-person fact, with no world attached to it and no guarantee of anything beyond the present act of thought. Descartes has found his stone. He has not yet found one other thing he is allowed to stand on. The whole rest of the system is the desperate work of getting from this single point back out to a world, and the next two chapters are the two beams he tries to build with: what kind of thing this thinking self is (chapter 4), and how to certify anything outside it at all (chapter 5).

Before he leaves the second Meditation, Descartes does one more thing that secretly settles the argument of the next chapter. He picks up a piece of beeswax. Fresh from the comb, it has a taste of honey and a smell of flowers; it is a certain color, a certain shape and size; it is hard and cold, and it makes a sound when rapped. Everything that lets one identify it seems to come through the senses. "But behold," he says, "whilst I am speaking, 'tis put to the Fire." The taste burns off, the smell vanishes, the color changes, the shape collapses, it grows liquid and hot, barely touchable, and it no longer makes any sound when struck. Every single sensory quality used to identify the wax is now gone, and yet there is no doubt that it is the same wax. So whatever lets one grasp the wax itself, the thing that persisted through the total change of all its sensible qualities, cannot have been the senses, because the senses reported only the qualities, and all of those changed. It must be the mind (the intellect) that grasps the wax as one continuing thing (something extended, flexible, changeable) underneath the flux of its appearances.

The wax does two jobs, and the second one is the quiet hinge of his whole metaphysics. The obvious job: it shows that our real knowledge of material things is intellectual, not sensory; the mind knows the wax, the senses only report its passing surface. (That is a direct strike against the Scholastic confidence that knowledge starts in the senses, and it is why the empiricists, two chapters of era 3 later, will come for it so hard.) The quiet job: the wax argument has just shown the thinking thing, the mind Descartes established a moment ago with "I am, I exist," doing something the body and the senses cannot do, grasping what persists when all the sensible qualities are stripped away. The intellect has a kind of access to reality the senses don't have. And that is the first hint of the great and terrible thesis of what follows: that the mind and the body are not just two parts of one person but two utterly different kinds of thing. The wax is where the mind is first shown working alone, and where dualism quietly begins.

Chapter 4

Mind and body

"Let us chuse for example this piece of Bees-wax... But behold whilst I am speaking, 'tis put to the Fire... Does it yet continue the same Wax?"

What Descartes actually claims here is one of the most caricatured ideas in the history of thought, and the caricature hides what is genuinely radical in it. His claim is that there are two completely different kinds of substance in a human being. "Substance" here is a technical word meaning a basic, self-standing kind of stuff: a thing that can exist on its own, depending on nothing else to be what it is. The first is the mind, which he calls res cogitans, "thinking thing": it thinks, and that is its whole nature; it is unextended (takes up no space), located nowhere in particular, not subject to the laws of mechanics, and indivisible (a thought cannot be cut in half). The second is the body, which he calls res extensa, "extended thing": it takes up space, has a location, obeys mechanical laws, and is divisible into parts, like any other chunk of matter. And his stunning assertion is that these two share not one single property in common. The mind has no size or shape or position; the body has no thoughts. They are as unlike each other as a number is unlike a stone. This is substance dualism.

The strangeness is the point. The act of reading a line of prose is an event in the mind, made of no matter, located nowhere one could point to, while the eyes tracking across the page are an event in the body, made of matter, located exactly there, at those coordinates, in that room. Descartes says those are not two sides of one thing. They are happenings in two utterly different kinds of stuff, with nothing in common, that nonetheless have to keep in perfect step every waking second of a life. The thought "turn the page" and the muscles that turn it belong, on his account, to two separate worlds that somehow run on the same track. That picture is what the whole chapter comes down to: whether it can possibly be true.

One potted summary needs correcting: "Descartes invented mind-body dualism." He did not. The idea that mind (or soul) and body are different things is ancient and was old news long before him. Plato's Phaedo separates soul from body explicitly: the soul is the higher part, imprisoned in the body, freed by death (the Plato read, and the Greeks chapter). Augustine spent the Confessions anatomizing the divided inner life of the self against the flesh (the faith-and-reason chapter). The Scholastics distinguished the two for a thousand years. If "dualism" just means "mind and body are distinct," Descartes is the heir to a very crowded tradition, not its inventor. Saying he invented it is simply false, and it is exactly the kind of textbook one-liner that scholarship rejects.

So what is new, because something genuinely is? Two things, and they have to be named precisely. First: two complete substances. In Plato, the soul and the body are the higher and lower parts of a single living creature; soul and body still belong to one person, related as better to worse, ruler to ruled. In Aristotle and the Scholastics, soul and body are form and matter, two aspects of one substance, not two substances. Descartes breaks that. His mind and body are each a complete substance in its own right, each able in principle to exist entirely without the other (the mind, he argues, could exist with no body at all), fitted together rather than fused. That is a sharper, more total separation than anything before him. Second, and this is the truly modern part: the body is a machine. Not metaphorically; literally. The human body, for Descartes, is a piece of clockwork, a mechanism of matter in motion, running by the same physical laws as everything else in the new mechanical universe. It would go on pumping blood and twitching its muscles, he argues, like a wound machine, even with no soul in it, the way a corpse's heart can beat on for a moment after death. The body is automated matter; the mind is the one thing that isn't. That mechanical body is genuinely new in the seventeenth century, and it is what makes Descartes' dualism explosive in a way Plato's never was.

And then comes the trap he builds for himself, the whole reason his dualism matters more than his predecessors'. By making the body so thoroughly a machine (a thing that moves only the way machines move, by contact and push and mechanical force, with no place in it for anything un-mechanical to get a grip) he makes it almost impossible to explain how the mind, which has no extension and no location and cannot push anything, could ever move it. Plato never had this problem in a sharp form: his soul inclines the body by desire, participates in the Forms, leans on the body without having to mechanically shove a clockwork. Descartes' precision about what the body actually is, pure mechanism, creates a problem his predecessors never faced. The crisp way to put it: Descartes didn't invent the idea that mind and body are different things. He invented the problem of how they connect, by making the body so completely a machine that no un-mechanical thing could obviously do anything to it. And that problem, as Elisabeth of Bohemia will show, is the one neither he nor anyone after him could solve.

He did try to answer it, and his answer is the detail everyone laughs at, so it deserves a fair hearing before the laughing. Descartes located the single point where mind and body meet at the pineal gland, a small structure deep in the center of the brain. (He treats this most fully in his final book, the Passions of the Soul of 1649, where the gland is called something like the principal seat of the soul, though the body is not, for Descartes, where the mind is; the mind being unextended is not in any place at all. The gland is just the point of contact, the lever, not the mind's address.) And his reason for choosing it is not stupid. The mind, he had argued, is one and indivisible; so it must interact with the body at a single, unified point, not be smeared across a divided organ. And the pineal gland is, almost uniquely in the brain, a single, unpaired structure, where nearly everything else (the two hemispheres, the pairs of nerves) comes in twos. So: one indivisible mind, one unpaired gland. Within his system, it is the logical choice. The pineal gland looks absurd only once one has already decided the whole dualist project is hopeless, which is exactly the objection Elisabeth of Bohemia delivers next, from inside his own physics.

One cost of this system follows so directly from the logic and is now so clearly wrong: Descartes held that non-human animals are automata, pure machines, without minds, and therefore without genuine feeling. They can behave as if they are in pain, recoiling from damage, because their bodies are built to react that way, exactly as a clock is built to strike. But there is, on his view, no one home to suffer. It follows ruthlessly from the dualism: if the mind is the thinking thing and animals do not think, then animals do not feel. The conclusion was contested in his own time and is regarded today as plainly false, and it shows the cost of the system, the price the clean two-substance picture exacts when it is run all the way out. A philosophy is partly known by what its premises force it to deny.

Chapter 5

God, the circle, and the princess

"This cognition, I think, therefore I am, is the first and most certain of all that occurs to anyone philosophizing in an orderly way."

Descartes is now in a corner of his own making. He has exactly one certainty (that he, a thinking thing, exists right now) and an evil genius still hypothetically rigging everything else. He has no world, no body he can trust, no mathematics he can rely on, because the demon could be faking even that. To get anything else back, he needs a way to disarm the demon, to establish that his clearest, most careful thoughts are not being systematically falsified. And his route to that, the bridge out of his own skull, runs through God. If he can prove that a perfect, non-deceiving God exists, then he can argue that such a God would not let him go hopelessly wrong about things he perceives "clearly and distinctly," and the demon dies, and the world can be rebuilt. Everything depends on the proofs of God. So he offers two.

The first proof (Meditation III) is sometimes called the trademark argument, a name that comes from Descartes' own image. The argument runs as follows. Within himself he finds an idea of God, an idea of an infinite, perfect being. Now, where could that idea have come from? Every idea, Descartes argues, must have a cause at least as great as what the idea represents (there can be no more in the effect than was in the cause). But I am finite and imperfect and constantly in error, and the idea of infinite perfection is not something a small, flawed, limited mind like mine could have manufactured out of its own resources, the way I can build a unicorn out of a horse and a horn I've already met. Infinite perfection isn't a patchwork of finite things I know; it's a positive idea of something with no limit at all, too big for me to be its source. So something actually infinite and perfect must have placed the idea in me, and that being is God. The idea of God in my mind is, as Descartes puts it, like the mark of the craftsman stamped on his work: a trademark left by the maker. Hence the name. In short: he carries an idea of infinite perfection that a small, finite, error-prone mind could never have manufactured, so something actually infinite and perfect must have placed it there.

The second proof (Meditation V) is a version of an old argument, the ontological argument, first made by Anselm six hundred years earlier (the faith-and-reason chapter). God is by definition a being with every perfection. Existence, Descartes argues, is itself a perfection: a perfect being that didn't exist would be missing something, and so wouldn't be perfect. Therefore existence belongs to God's nature the way having three angles that sum to a straight line belongs to a triangle's nature; one cannot consistently think of a supremely perfect being and deny that it exists, any more than one can think of a triangle whose angles don't add up. So God exists; a perfect being that didn't exist wouldn't be perfect, so once the meaning of "perfect being" is really grasped, existence comes built in. Both proofs do the same job in the system: they re-establish a non-deceiving God, which then certifies that what I perceive clearly and distinctly is true in general, and that rule, "whatever I perceive clearly and distinctly is true," is the engine that lets Descartes rebuild mathematics, the external world, his own body, everything. God is the bridge from the lonely cogito back out to a knowable world.

And the bridge has a crack in it that was spotted before the book was even properly in print, a crack so clean it has its own name, the Cartesian circle. Descartes uses God to guarantee that his clear and distinct ideas are reliable (a good God wouldn't deceive me about what I see clearly). But the proofs of God's existence themselves depend on reasoning that Descartes accepts only because it seems clear and distinct to him. So: clear-and-distinct ideas are trustworthy because God exists, but God's existence is established by clear-and-distinct ideas. The thing that is supposed to be the guarantee (God) is reached only by trusting the very faculty (clear and distinct perception) it was brought in to guarantee. It runs in a circle. The whole rebuilt world hangs from a loop with no anchor.

The history of the objection is more specific than the era-3 read named. The circle was raised first by Marin Mersenne (the friar at the center of Europe's scientific correspondence, who had circulated Descartes' manuscript to the sharpest minds of the day before publication) in the set of Second Objections printed with the Meditations. It was then pressed harder, with more philosophical precision, by the theologian Antoine Arnauld in the Fourth Objections. (These objections, and Descartes' replies, were printed in the same volume as the Meditations from the 1642 edition onward, a remarkable thing in itself: Descartes published the strongest attacks on his own book bound together with the book, the way Plato staged the demolition of his own theory of Forms in the Parmenides, or Aquinas opened with the best arguments against God before answering them. Honest minds arm their critics.) Descartes replied to the circle; most scholars agree he never satisfactorily escaped it. It remains a live problem. He had reset the question for the whole modern era. He had not closed his own proof.

But the deepest objection of his life was not about God. It was about the seam between mind and body from the last chapter, and it came from a woman the textbooks spent three centuries miscataloguing as a royal pen-pal. Princess Elisabeth of Bohemia (1618–1680) was a daughter of Frederick V, the briefly-king of Bohemia whose defeat helped touch off the Thirty Years' War; the war scattered her family into exile, and she grew up sharp, learned, and displaced, at The Hague. She was not a fan writing to a great man. She had read Descartes' physics closely enough to find the exact load-bearing beam it could not carry, and in a letter of 6 May 1643 she put a question to him that has never been answered.

Her objection (because no public-domain English of her letters has been confirmed, it is faithfully restated here, never quoted) runs like this. In his own physics, she points out, one body makes another body move only by pushing it: by contact, by being extended and touching the extended thing it shoves. That is the entire mechanism his mechanical universe allows. But he had just argued that the mind is not extended at all: it takes up no space, has no surface, touches nothing. So by what possible mechanism does this unextended mind get an extended body to move? How does a thought, which has no location and cannot push, lift an arm? Everything his physics permits as a cause is denied to his mind by his own definition of it. So how, precisely, does the mind move the body? This is the interaction problem, and it is not a clever quibble around the edges. It is aimed at the exact center of the system, at the very seam Descartes had marked with the pineal gland. She did not ask how the gland works. She asked how anything unextended could move anything extended at all, which is to ask whether the two-substance picture can possibly deliver what every waking second requires of it.

Descartes' reply was weak, and Elisabeth caught it instantly, and the way she caught it is the measure of her as a philosopher. He reached, in effect, for an analogy: people experience the soul moving the body, he suggested, much as they have always experienced heaviness (pesanteur, weight) seeming to pull a heavy object straight down toward the earth, as if by some quality inside it drawing it down. The mind is simply felt to act on the body; treat it as a brute given the way weight was once treated. And Elisabeth pounced, because she knew his own work too well: heaviness as an inner pulling-quality was exactly the kind of occult Scholastic explanation that his entire new mechanical physics was built to abolish. He had taught a generation to throw out mysterious inner "qualities" that things supposedly have to make them behave; bodies fall, on his physics, because of mechanical pushes, not because of a hidden downward-tugging power. And now, to save his theory of the mind, he was reaching back for precisely the kind of explanation he had spent his career dismantling. He had no good answer for her. He never gave one, not in the correspondence, not anywhere, and the soundest verdict, which most scholars share, is that there is no good answer inside his system.

Elisabeth of Bohemia did not pose a curious question to a famous man who graciously addressed it. She identified the structural flaw in the most influential philosophical system of the seventeenth century (the wound in it) and she did it with such precision, from inside his own physics, that he could not close it and his successors built whole new universes to get around it. He valued the exchange enough to dedicate his last book, the Passions of the Soul, to her. She is not a sidebar to Descartes. She is the thinker who found the crack he spent the rest of his life unable to seal, and the crack is the subject of the final chapter.

Chapter 6

The afterlife of a wound

"I think, therefore I am."

Descartes died in Stockholm in 1650 having left two great gifts to philosophy and one great wound, and the next two centuries are the story of what everyone did with all three. The gifts: he had made the first-person standpoint the starting place of philosophy, and he had insisted that knowledge be rebuilt as a system, from certain foundations, by necessary steps, like a geometry of everything. The wound: he had split mind and body into two complete substances and then could not say how they touch. Every major philosopher who came after him in this era is, in one way or another, answering Descartes: building on his method, or recoiling from his confidence, or trying to heal the wound Elisabeth had opened. The whole rest of era 3 is a set of responses to this one man.

The rationalists built on him. Each one took the interaction problem as a starting point and dealt with it by refusing one of Descartes' premises. Spinoza dissolved the problem: instead of two substances that have to interact, he argued there is only one substance in the universe (he called it God, or Nature), and that mind and body are not two things at all but two ways of describing the very same single thing, so the question "how do they interact?" never arises, because they were never two things to begin with (era 3). Leibniz circumvented it: he filled the universe with infinitely many simple substances ("monads") that, he said, never actually interact; they only seem to, because God set them all running in perfect harmony at the first instant, like countless clocks wound to keep the same time forever, so a decision and an arm's motion match without one ever touching the other. Malebranche (Nicolas Malebranche, a French Oratorian priest and philosopher, 1638–1715) delegated it to God directly: on his view (called occasionalism) the mind never really moves the body at all; God does, taking each human decision as merely the occasion on which He moves the limb. Three of the greatest systems of the age, and each one is, in part, a different escape route from the corner Descartes painted himself into. The wound was that productive.

The empiricists reacted against him, against the other gift, his towering confidence that reason alone, working inside a single mind, could reach substantial truths about God, the soul, and the world. John Locke went after Descartes' innate ideas: where Descartes held that the mind comes furnished with certain ideas it never learned (the idea of God, of perfection, of mathematical truth), Locke argued the mind starts as blank paper, with nothing written on it that experience did not put there (era 3). And Hume, at the very end of the era, took the empiricist knife all the way to the handle: he applied it to exactly the things Descartes had been most sure reason could deliver (the existence of God, the substantial self, the necessity of cause and effect) and concluded that none of them survive honest scrutiny. The lonely thinking "I" that Descartes had made the unshakable foundation of everything, Hume could not even find when he looked inside for it; he found only a bundle of passing perceptions and no "self" underneath them. The era that opened with Descartes certain a single mind could doubt its way to bedrock closed with Hume showing, politely and devastatingly, that the same mind, followed honestly, cannot prove the sun will rise tomorrow.

And the wound itself, the mind-body problem, the question Elisabeth pressed and Descartes could not answer, never closed. This is the honest and slightly astonishing fact to end on. Spinoza's one-substance answer, Leibniz's harmony, Malebranche's God-does-it, the materialists' "there is no immaterial mind to connect in the first place": every one of these is a way of getting around the problem rather than solving it on Descartes' own terms, and none of them ever became the settled answer. Four hundred years later, the question of how conscious experience relates to the physical brain (how the thing it is like to see red connects to the firing of neurons) is still one of the genuinely open problems in philosophy, argued in almost the form Elisabeth handed Descartes in 1643. He did not solve it. He may even have made it permanent: by drawing the line between mind and matter as sharply as he did, by making the body so completely a machine, he carved the modern version of the problem into the shape it still has. Philosophy has not gotten past it. The cleared site he made when he shut himself in that warm room and resolved to doubt the world is still the ground it stands on.

So measure the man the way he would have wanted to be measured: by what he changed, honestly, warts kept in. He did not, in fact, prove God; the Cartesian circle was named before the ink dried, and he never escaped it. He did not solve the mind-body problem; Elisabeth showed within months that he couldn't, and four centuries have not. He suppressed his own physics out of a clear-eyed fear of the Church, so his fullest system never saw print while he lived. His dualism forced him to deny that animals can feel, which we now know is simply wrong. He got a great deal wrong. And yet: he reset the question of the entire modern era, from "what does the authority tell us?" to "what can I, examining my own mind, establish from nothing?", and everyone who came after, the rationalists building on him and the empiricists tearing at him and Kant a generation later trying to adjudicate the whole quarrel, was working on ground he cleared. He tried to burn down everything he believed and rebuild it on a single certainty he could not doubt. He found the certainty. He never finished the building. But the foundation he found, I think, therefore I am, is still the first stone anybody reaches for when they want to start philosophy over from the beginning, which is about the highest thing that can be said of a four-word sentence: four hundred years on, we still cannot get behind it, and we still cannot build the whole house on it. That is the inheritance. A floor that holds, and a house that won't quite stand on it, and the next era (Kant, trying to referee the fight Descartes started, era 4) picking up exactly where the demolition left off.