The Latin phrase

The Latin phrase cogito ergo sum (trans. "I think, therefore I am") is possibly the single best-known philosophical statement and is attributed to René Descartes. The argument that is usually summarized as "cogito ergo sum" appears in Meditations on First Philosophy, in which Descartes attempts to build an entire philosophical system from scratch, with no prior assumptions. In order to begin this undertaking, he reasons that since all his beliefs were derived from potentially misleading sense data or potentially fallacious logic, he would trust nothing he had previously taken to be true. That is to say, he would systematically doubt all that could conceivably be doubted. However, this leads him to discover that the one thing that he cannot doubt is his own existence. After all, he claims, something nonexistent is incapable even of the act of doubting. Thus the formulation, "I think, therefore I am", was the starting point of his philosophy. Although the ideas expressed in cogito ergo sum are most commonly associated with Descartes, they were present in many of his antecedents, especially Saint Augustine in De Civitate Dei (books XI, 26) who makes this argument, and anticipates modern refutations of it. See, Principia Philosophiae, §7: "Ac proinde haec cognitio, ego cogito, ergo sum, est omnium prima et certissima etc."