René Descartes (1596–1650) uses methodic doubt as demolition equipment: if a belief can be coherently doubted, temporarily set it aside and ask what, if anything, remains certain.

The famous result is the cogito: while doubting, thinking is occurring; therefore the existence of the thinking subject cannot be doubted in that act — conventionally summarized as cogito, ergo sum, “I think, therefore I am.”

Big idea: Descartes does not argue that nothing exists. He uses extreme skepticism as a method for finding a foundation that survives skepticism.

His distinction between mind (thinking substance) and matter (extended substance) creates the classic Cartesian mind–body problem: if mind is not spatial matter, how can mental intention and physical body interact? Descartes proposed an interactionist dualism, but the explanatory problem became more famous than his proposed solution.

Descartes is also a foundational figure in analytic geometry and mechanistic natural philosophy. “Cartesian” therefore describes much more than mind–body dualism.

A common mistake is to treat the cogito as a syllogism requiring the hidden premise “everything that thinks exists.” Descartes presents it more as an immediately self-verifying act: trying to doubt one's current thinking confirms that thinking is happening.