Descartes wanted certainty, and he was willing to tear everything down to find it. In his Meditations, he adopts a method of radical doubt: he will treat as false anything he can possibly doubt, to see whether anything at all survives the demolition. The senses sometimes deceive us, so he doubts them. Dreams can feel real, so perhaps he is dreaming now. He even imagines an all-powerful deceiver bent on fooling him about everything, including mathematics. One by one, his beliefs fall.
Then he hits bedrock. However powerful the deceiver, however deep the doubt, there is one thing Descartes cannot doubt: that he is doubting. And to doubt is to think, and to think, something must exist to do the thinking. Even if he is wrong about literally everything else, the very act of thinking proves the existence of a thinker. This is the cogito — 'I think, therefore I am' — the single certain point from which he sets out to reconstruct knowledge on secure foundations.
Mind and bodyThe dualism that still haunts us.
From the cogito, Descartes drew a fateful conclusion about what he is. He knows he exists as a thinking thing before he can be certain his body exists at all. This led him to argue that mind and body are two fundamentally different kinds of substance: the mind, an immaterial thinking thing, and the body, a physical extended thing. This mind–body dualism gave philosophy the 'mind–body problem' — how something immaterial could interact with something physical — which remains one of the deepest unsolved puzzles about consciousness.
Descartes was also a great mathematician, whose work helped unite algebra and geometry, and his ambition to ground all knowledge in reason makes him a founder of rationalism. Not everyone accepted his conclusions; later philosophers challenged both the cogito and the dualism, and the mind–body problem he bequeathed is debated as fiercely as ever. But by insisting that we rebuild knowledge from what we can be certain of, and by putting the thinking self at the centre, Descartes opened the era we still call modern philosophy.