René Descartes

Descartes is the hinge between medieval and modern philosophy. He decided to demolish every belief he had ever held and rebuild knowledge from the ground up. In the Meditations on First Philosophy he sits alone by a fire and asks: what if everything I sense is a dream? What if a powerful evil demon is deceiving me? Can anything survive maximal doubt?

One thing does. To doubt is to think, and to think is to be a thinking thing. Cogito, ergo sum — I think, therefore I am — is the bedrock he finds when everything else has been swept away. From this single rock he tries (less convincingly) to rebuild knowledge of God and the external world.

He also invented analytic geometry, formulated Cartesian coordinates, and proposed a dualist account of mind and body that we are still struggling to escape. Modern philosophy is, in many ways, the long argument with Descartes' picture of a thinking self set against a mechanical world.