http://en.wikipedia.org/wiki/Cogito_ergo_sum

Cogito ergo sum

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"Cogito, ergo sum" (English: "I think, therefore I am"), sometimes misquoted as Dubito, ergo cogito, ergo sum (English: "I doubt, therefore I think, therefore I am")[1], 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).[2]

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: "Ac proinde hæc cognitio, ego cogito, ergo sum, est omnium prima & certissima, quæ cuilibet ordine philosophanti occurrat.", by which time it had become popularly known in the English speaking world as 'the "Cogito Ergo Sum" argument', which is usually shortened to "Cogito" when referring to the principle virtually everywhere else.