http://en.wikipedia.org/wiki/Meditations_on_First_Philosophy#Me ... In other words, one's consciousness implies one's existence. In one of Descartes' replies to objections to the book, he summed this up in the phrase, "I am, I exist", which is often confused with the famous quote, "I think, therefore I am". ... http://en.wikipedia.org/wiki/Discourse_on_the_Method The Discourse on Method is best known as the source of the famous quotation "Je pense, donc je suis" ("I think, therefore I am"), which occurs in Part IV of the work. (The similar statement in Latin, Cogito ergo sum, is found in §7 of Principles of Philosophy.)