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 existence. The success of his attempt to achieve certainty can be analysed in relation to how far he overcomes the universal doubt he has…