“Cogito, ergo sum” (English: “I think, therefore I am”) . . . is a philosophical statement in Latin used by René Descartes, which became a foundational element of Western philosophy. The simple meaning of the phrase is that if someone is wondering whether or not he exists, that is in and of itself proof that he does exist (because, at the very least, there is an “I” who is doing the thinking).
Descartes’s original statement was “Je pense donc je suis,” from his Discourse on Method (1637). He wrote it in French, not in Latin, thus reaching a wider audience in his country than that of scholars. He uses the Latin “Cogito ergo sum” in the later Principles of Philosophy (1644), Part 1, article 7.
At the beginning of the second meditation [in Meditations on First Philosophy], having reached what he considers to be the ultimate level of doubt — his argument from the 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, the tool he uses to stop himself sliding back into ungrounded beliefs), his belief in his own existence would be secure, for how could he be deceived unless he existed in order to be deceived?
According to many of Descartes’ specialists, including Étienne Gilson, the goal of Descartes in establishing this first truth is to demonstrate the capacity of his criterion — the immediate clarity and distinctiveness of self-evident propositions — to establish true and justified propositions despite having adopted a method of generalized doubt. As a consequence of this demonstration, Descartes considers science and mathematics to be justified to the extent that their proposals are established on a similar immediate clarity, distinctiveness, and self-evidence that present itself to the mind. The originality of Descartes’ thinking, therefore, is not so much in expressing the cogito . . . but on using the cogito as demonstrating the most fundamental epistemological principle, that science and mathematics are justified by relying on clarity, distinctiveness, and self-evidence.