Descartes

Descartes is the founder myth of modern philosophy — the man who allegedly cleared the deck of medieval scholasticism and started over from first principles. The story is too clean (medieval philosophy didn't actually go quietly), but the rhetorical move was real: take nothing on authority, doubt everything that admits of doubt, and see what remains.

The *Meditations on First Philosophy* (1641) walks you through it. Could you be dreaming? Possibly. Could a malicious demon be deceiving you about all your sensory experience? Possibly. But — and this is the move — could that demon deceive you about the fact that you are thinking? No: even doubt is a kind of thought. *Cogito, ergo sum*. I think, therefore I am. From this fixed point Descartes tries to rebuild the edifice: a non-deceiving God, the reliability of clear and distinct ideas, the external world.

The rebuild is less convincing than the destruction. The famous "Cartesian circle" — using clear and distinct ideas to prove God, then using God to validate clear and distinct ideas — has been debated for nearly four centuries. But the dualism that came out of it shaped everything: mind and body as two fundamentally different substances, interacting (somehow) at the pineal gland. The "mind-body problem" in contemporary philosophy of mind is in many ways the long shadow of Descartes' division.

He was also a working mathematician — Cartesian coordinates, optics, mechanics. The philosophy didn't sit apart from the science; it was supposed to give the new science its epistemological warrant. Reading the *Meditations* alongside the *Principles of Philosophy* and the *Discourse on the Method* gives you the full picture: a mind trying to give the new physics a foundation that scholastic Aristotelianism couldn't.