Descartes's original statement was "Je pense donc je suis," from his Discourse on Method (1637). He wrote it in French, not in Latin, thereby reaching a wider audience in his country than that of scholars. He uses the Latin "Cogito ergo sum" in the laterPrinciples of Philosophy (1644), Part 1, article 7: "Ac proinde hæc cognitio, ego cogito, ergo sum, est omnium prima & certissima, quæ cuilibet ordine philosophanti occurrat." (English: "This proposition, I think, therefore I am, is the first and the most certain which presents itself to whoever conducts his thoughts in order".). At that time, the argument had become popularly known in the Englishspeaking world as 'the "Cogito Ergo Sum" argument', which is usually shortened to "Cogito" when referring to the principle virtually everywhere else.
The phrase Cogito ergo sum is not used in Descartes' Meditations on First Philosophy, but the term "the cogito" is (often confusingly) used to refer to an argument from it. Descartes felt that this phrase, which he had used in hisearlier Discourse, had been misleading in its implication that he was appealing to an inference, so he avoided the word ergo and wrote "that the proposition, I am, I exist, is necessarily true whenever it is put forward by me or conceived in my mind." (Meditation II.)
At the beginning of the second meditation, having reached what he considers to be the ultimate level of doubt — his argument fromthe existence of a deceiving god — Descartes examines his beliefs to see if any have survived the doubt. In his belief in his own existence he finds it is impossible to doubt that he exists. Even if there were a deceiving god (or an evil demon), one's belief in their one's existence would be secure, for how could one be deceived unless one existed in order to be deceived?
But I have convincedmyself that there is absolutely nothing in the world, no sky, no earth, no minds, no bodies. Does it now follow that I too do not exist? No. If I convinced myself of something [or thought anything at all] then I certainly existed. But there is a deceiver of supreme power and cunning who is deliberately and constantly deceiving me. In that case I too undoubtedly exist, if he is deceiving me; and lethim deceive me as much as he can, he will never bring it about that I am nothing so long as I think that I am something. So, after considering everything very thoroughly, I must finally conclude that the proposition, I am, I exist, is necessarily true whenever it is put forward by me or conceived in my mind. (AT VII 25; CSM II 16–17)
There are three important notes to keep in mind here. First,he only claims the certainty of his own existence from the first-person point of view — he has not proved the existence of other minds at this point. This is something that has to be thought through by each of us for ourselves, as we follow the course of the meditations. Second, he is not saying that his existence is necessary; he is saying that if he's thinking, then necessarily he exists (seethe instantiation principle). Third, this proposition "I am, I exist" is held true not based on a deduction (as mentioned above) nor on empirical induction, but on the clarity and self-evidence of the proposition.
Descartes does not use this first certainty, the cogito, as a foundation upon which to build further knowledge; rather, it is the firm ground upon which he can stand as he works to...