Descartes

René Descartes

Try the experiment he actually proposed, because the famous sentence is worthless until you do. Shut the door, sit down, and resolve to doubt everything you can possibly doubt. The senses go first: they have fooled you before, so anything they report could be wrong. The solid room around you goes next, because you have dreamed rooms this convincing and believed them, so how can you be sure you are not dreaming now? Then push past even that. Two and three making five could be the work of some immense deceiver who rigs your mind to slip every time you count. Descartes runs this demolition not because he doubts, for he is among the least skeptical of the great philosophers, but because he is hunting bedrock. He wants to find whether anything at all can survive the most violent doubt he can manufacture, and then to build only on that. The Meditations on First Philosophy are written in the first person and meant to be performed, one a day, the way a monk performs a spiritual exercise. The point was never the slogan. The point was the procedure, and the procedure still works.

The deliberate demolition. What makes Descartes the founder of modern philosophy is not any single conclusion but this method, doubt turned into an instrument. The rule is to treat whatever is merely uncertain as if it were outright false, so that anything still standing at the end is not just probable but unshakeable. He had set out the working version years earlier, in the Discourse on the Method, as four plain maxims: accept nothing as true that you do not clearly and distinctly see to be so; break every hard problem into as many smaller problems as it takes; order your thinking from the simplest objects upward to the most complex; and review the whole chain so often that nothing can slip out of it unnoticed. It reads like an engineer's checklist because that is what it is. Behind it stands a young soldier who, by his own account, spent a winter's day in 1619 shut in a warm room with nothing to do but think, and came out convinced that the inherited sciences were a city of crooked medieval streets that ought to be levelled and rebuilt by one planner on a single clear plan. He spent the rest of his life being that planner.

What survives. One thing, it turns out, cannot be torn down. Suppose the deceiver is as powerful and as malicious as you like, rigging your senses, your memories, even your arithmetic. He still cannot trick you into believing you exist when you do not, because the trick would need someone there to be tricked. The instant you doubt, you have confirmed a doubter. I am, I exist: necessarily true, Descartes says, every time you think it or conceive it. Notice what kind of certainty this is. It is not the conclusion of an argument, not "everything that thinks exists, I think, so I exist," because that would lean on a general premise the doubt has not yet earned back. It is performative, true in the doing and only in the doing, won again each time it is thought. Descartes did not pull the move from nowhere. Augustine had written si fallor, sum, if I am deceived, I exist, eleven centuries earlier; the Persian philosopher Avicenna had imagined a man created floating in the dark, sealed off from every sensation, who would still know that he was. What Descartes did that they had not was to make this single fixed point the place from which an entire reconstruction of knowledge could be levered up.

The rebuilding, and its crack. From that one certainty Descartes levers the rest back into being. Among his ideas he finds one he is sure he could not have manufactured, the idea of an infinite and perfect being, too large to have issued from a finite and imperfect mind; that idea, he argues, must have a cause as great as itself, so a perfect being exists, and a perfect being would not stoop to deceive. With a non-deceiving God underwriting him, he can trust whatever he perceives clearly and distinctly, and so wins back mathematics, the external world, the body. The reconstruction is audacious, and it carries a famous flaw: he uses clear and distinct perception to prove God, then turns around and uses God to guarantee clear and distinct perception, and that circle has been turning in seminar rooms ever since Antoine Arnauld traced it. What is remarkable is that Arnauld traced it inside Descartes's own book. Descartes circulated the Meditations before publishing and printed six sets of objections from the sharpest readers he could find, Hobbes and Gassendi and Arnauld among them, with his replies, all bound in alongside the argument they attack. He set the case and its cross-examination between the same two covers. It is one of the most honest things a philosopher has ever done, and the exact opposite of the dogmatist the caricature makes of him.

Mind and body, and the problem he knew. Here the cartoon does him the most damage. Yes, Descartes argues that mind and body are two really distinct substances, the mind essentially a thing that thinks, the body essentially a thing that takes up space, each able in principle to exist without the other. The textbook stops there and calls it naive, a ghost loosely haunting a machine. But Descartes saw the hard part more clearly than many of his critics. If the two are so utterly different, how do they ever touch? How does a decision, which has no size and no place, move an arm, which has both? He guessed at a meeting point in the pineal gland and was visibly unsatisfied with the guess. When Princess Elisabeth of Bohemia put the question to him directly in 1643, pressing him on how an unextended mind could possibly push an extended body, he never produced a clean answer, and his late Passions of the Soul is in large part an effort to think the union of mind and body rather than merely their distinction. He handed us the mind-body problem not because he was blind to it but because he was the first to feel its full weight.

The world as mathematics. The other half of Descartes is easy to lose if you meet him only as the doubting metaphysician: he was a practising scientist who wanted to rewrite physics as geometry. In his world a body has no qualities but extension, shape, size, and motion, and everything else, colour, warmth, the apparent purposes of things, is only how those motions register in a mind. An animal is an exquisite automaton; the acorn no longer strives toward the oak it will become, because nothing in nature strives toward anything at all. This quietly demolished the Aristotelian physics that had governed Europe for four centuries, in which every natural thing had a built-in end, and put in its place a universe of matter in motion to be calculated rather than read for meaning. The same cast of mind had already produced his most lasting single stroke. By laying two number lines across each other he turned geometry into algebra, so that a curve became an equation and a shape could be computed; every graph with an x and a y axis is Cartesian still. His deepest conviction was that the qualitative could be made measurable, and modern science took him at his word.

A method you can still run. The room reads Descartes in John Veitch's fluent Victorian translations of the Discourse and selections from the Principles; the Meditations are discussed from the wider record rather than quoted line by line, since that translation is not yet in the corpus, and the chat is candid about the difference and cites by work and part. What carries over from his century to ours is the method, and it is genuinely usable. Treat it as a daily discipline for thinking under uncertainty: clear away whatever merely seems so; find the few things you actually see clearly and distinctly; build outward only from those; cut every tangled question into parts small enough to settle one at a time; lay them out simplest first; and run back over the chain often enough that nothing slips. It will not hand you Descartes's God or his two substances, both of which his successors took apart. It hands you something he prized more, a way of starting any hard problem from the ground rather than from the rumour of the ground.

What Descartes really founded was not a doctrine but a problem everyone after him had to answer. Spinoza decided the two substances were one substance seen two ways; Leibniz split reality into countless windowless minds running in pre-set harmony; Locke and Hume went after the innate ideas at the root of the system; Kant rebuilt from the ground the whole question of how a thinking self can know a world outside it; Husserl, three centuries on, restarted philosophy with a book he simply called Cartesian Meditations; and when Gilbert Ryle set out to bury the dualism for good, he still had to fight Descartes to do it. That is the mark of a founder: not that he was right, but that no one who came after could find a way around him. He taught that doubt, used on purpose and then defeated, is not the enemy of knowledge but the only honest way to lay its foundation. Centuries of arguing with him have not improved on the lesson.