Published in Latin in Paris and Amsterdam in 1641, the *Meditations on First Philosophy* is Descartes' radical thought experiment in six brief meditations. The method is simple and runs as follows: reject as if false anything about which the slightest doubt is possible — and see what is left that cannot be doubted.

The senses deceive. Even mathematics could in principle be the work of an 'evil demon'. But within all this doubt there must be a doubter — a thinker. *Cogito, ergo sum.* This point becomes the foundation on which any subsequent edifice of knowledge can be built.

Descartes proceeds from there to reconstruct the existence of God and the reality of the physical world; those proofs are today regarded as more contested. But the method itself — systematic doubt, the placement of the subject at the centre of knowledge, the dualism of mind and body — set the direction of Western philosophy for the next three centuries. Spinoza, Locke, Hume, Kant, and eventually phenomenology and analytic philosophy were all forced to reckon with the questions these meditations opened.

Descartes' picture of the disembodied mind (the famous 'Cartesian dualism') remains the central problem that philosophy of mind still tries to move past. 1641 holds a symbolic position as the starting date of modern philosophy.