H. P. Grice, "Cartesianum implicatum"

Cogito; ergo, sum. COGITO; ERGO, SUM -- cogito ergo sum Metaphysics, epistemology, philosophy of mind [Latin, I think, therefore I am] The first principle or first truth of Descartes’s metaphysical system. I can doubt everything, including whether I have a body. But as long as I am engaged in the process of thinking, I exist. Even if I doubt my existence, there must exist an “I” who can doubt. It would be a contradiction to deny the existence of something that is thinking. Thus this proposition is certain and indubitable. It is the first limitation to the agnostic doubt, and the starting-point of strict knowledge. It implies, of course, some prior knowledge of the meaning of the terms involved and their logical implications, but it is the first matter of existence of which one can be sure. The proposition might be construed syllogistically as presupposing a major premise that everything that thinks exists. But Descartes emphasized that the certainty of my existence is not a logical inference; rather it is an individual and immediate act of thinking. “Observing that this truth ‘I am thinking, therefore I exist’ (Ego cogito ergo sum) was so firm and sure that all the most extravagant suppositions of the sceptics were incapable of shaking it, I decided that I could accept it without scruple as the first principle of the philosophy I was seeking.” Descartes, Discourse on Method