Can Descartes be certain that he is thinking? How? Can he be certain that he exists? How? (And who is he?)

Descartes' statement "I think therefore I exist" raises questions about the meaning of thought, the meaning of existence but most fundamentally, in what sense he can be certain. The difficulty in establishing the certainty of "I think" and "I exist" is that the two concepts are interrelated. Thus, for example, differing interpretations of what it is to think will have a profound impact on the question of whether Descartes can achieve the certainty of his …

…existing. In discourse four, he expresses this when he says that there is nothing that assures him he is speaking the truth other than conceiving "very clearly and very distinctly." Thus Descartes is unable to claim to become certain of his own existence on the basis of the method of doubt expressed in the first meditation. He is forced to rely upon (if naturally intuitive) presuppositions in terms of being certain of his own being.