I Think, Therefore I Am

Descartes was born into a collapse. Copernicus had moved the earth out of the centre of the universe. Religious war had made competing authorities lethal rather than merely disputed. And ancient scepticism had been revived by Montaigne with enough force that educated people were genuinely unsure whether anything could be known at all.

His response was drastic and, in its way, very modern: stop appealing to authority entirely and rebuild from the ground. Not by examining beliefs one at a time, which would take forever, but by attacking whole sources at once. If a source can be doubted, everything resting on it goes, provisionally, in one move.

The first source is the senses. They demonstrably deceive (a straight oar looks bent in water) and a source that has lied once cannot be trusted absolutely. But that only removes some beliefs, so he pushes harder. Right now you are certain you are awake. You have also been certain in dreams. Name a test you could run, from inside the experience, that distinguishes the two. There is none. So the whole sensory world goes.

Even that is not enough, because some things survive dreaming. Two plus three makes five whether you are asleep or not; a square has four sides in any state of consciousness. To reach these he introduces the device he is most famous for: suppose a deceiver of enormous power is devoting itself to fooling you, faking not just the world but the very steps of your reasoning. Under that supposition, arithmetic goes too.

The demon is a tool, not a belief. Descartes does not think one exists. Its job is to push doubt to the absolute limit, on the principle that whatever survives the worst case survives everything.

And something does survive. To be deceived, you must be something. The demon can falsify the content of every thought and cannot make it the case that nothing is thinking while thinking is occurring. In the Discourse this becomes the famous line, I think therefore I am. In the Meditations he puts it more carefully: I am, I exist, is necessarily true whenever I assert it.

Two things about the cogito are routinely got wrong. It is not a syllogism with the suppressed premise that everything which thinks exists: Descartes insists it is grasped in a single act, not deduced. And it is available only in the first person and only while you are actually doing it. It proves that a thinker exists, to that thinker, at that moment. It is a very small piece of ground.

He then builds outward, and this is where most readers stop following him: from the cogito to God, from God's non-deception to the reliability of clear and distinct perception, and from there back to a physical world. Arnauld pointed out immediately that the reasoning looks circular, since clear and distinct perception is used to prove God and God is then used to license clear and distinct perception. Descartes distinguishes perceptions held right now from remembered ones. Few readers have found the distinction sufficient.

There is a sharper problem, and it came from Princess Elisabeth of Bohemia in correspondence. Descartes has concluded that mind and body are two different kinds of substance: one unextended and indivisible, one extended and divisible. So how does the unextended one move the extended one? Causation seemed to require contact, and a thing with no extension cannot make contact with anything. His answer involved the pineal gland and convinced nobody, including him. That question is still the central problem in philosophy of mind, and every modern debate about consciousness is a descendant of it.

The demon has aged best of all. Ask whether we might be in a simulation and you are running Descartes' device on different hardware.