Discourse on Method

A stove-heated room in Germany, in the dead of a winter he is mostly waiting out: that is where Descartes resolves to doubt everything he has ever believed, all at once, the way you might raze a house down to its foundations rather than patch the walls. The senses deceive, and dreams counterfeit the waking world so exactly that nothing inside one ever announces itself as false; even mathematics, that last redoubt of certainty, might be corrupted by some flaw built into his own mind. He follows the doubt all the way down and finds, at the very bottom, one thing it cannot dissolve: the fact of his own doubting. I think, therefore I am becomes the single plank sound enough to build a new world on. What still astonishes is the plainness of the telling: he wrote it in French, not the Latin of the schools, so that any reader with reason enough to follow could watch a mind take itself apart in public and, sentence by sentence, put itself back together.