Descartes went on a deconstruction trip, proving that he knew almost nothing with absolute certainty. He proved that his memory was sometimes wrong, and sometimes he dreamed and thought it was reality--reality was conceivably a dream (or a TV show). He said that his faculty for logic was not perfect. He concluded "I think, I am." (Later phrased I think, therefore I am.) Of course, this was logic, and he had already stated his logic could be flawed. . .