Rationalism senses. For instance, his famous dictum, "cogito ergo sum" or "I think, therefore I am", is a conclusion reached "a priori" i.e., prior to any kind of experience on the matter. The simple meaning is that doubting one's existence, in and of itself, proves that an "I" exists to do the thinking. In other words, doubting one's own doubting is absurd. This was, for Descartes, an irrefutable principle upon which to ground all forms of other knowledge. Descartes posited a metaphysical dualism, distinguishing between the substances of the human body (""res extensa"") and the mind or soul (""res cogitans""). This
Cogito and the History of Madness at the end of the renaissance and the beginning of the classical age. He then discusses the treatment of madness by Descartes in the "Meditations on First Philosophy". Foucault argues that Descartes refuses to recognise a commonality between himself and people who are mad and uses this dissociation as an excuse not to take madness seriously as a grounds for doubt. For Foucault this exclusion of madness by Descartes leads to a cogito that is more or less arbitrarily self-assured of its own rationality. Derrida argues that Descartes might appear to dismiss madness at the point of the meditation to