This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
Descartes' method in the Meditations is not ordinary doubt, the sort that questions a particular claim because of a particular reason to distrust it. It is a deliberate, total doubt, applied to every belief he holds, including the ones that seem most obviously secure, on the ground that if even one of his beliefs might be false and he cannot yet tell which, none of them can serve as a foundation until sorted. The senses fail sometimes, so anything resting on the senses goes. Perhaps he is dreaming right now and only seems to be seated by a fire holding paper, so even the vivid conviction of waking experience goes. He pushes further still, imagining a deceiver of unlimited power bent on making even two plus three seem to equal five when it does not, so that even mathematics, which had seemed doubt proof, goes as well. What is left, after every possible ground for doubt has been applied, is a single point that survives the very act of doubting it: whatever else might be false, the doubting itself is happening, and something must be doing the doubting. I think, therefore I am is not a conclusion reached by inference from other premises, on Descartes' own account, but something closer to an immediate recognition, true the instant it is entertained, because entertaining it already proves a thinker exists to entertain it. From that one certain point, Descartes tries to rebuild everything else, God's existence, the reliability of clear and distinct ideas, the external world, by reason alone. Whether the rebuilding succeeds is a separate question from whether the doubt was total enough to need it; but the method, doubt everything until you strike something doubt itself cannot touch, is the founding gesture of Rationalism as a program, not merely one philosopher's peculiar exercise.