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The original quote "I think, therefore I am" is the thinking man's way of distinguishing himself from your average citizen, the mindless zombie.
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I don't think it's a quote to distinguish himself from normal people.
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The argument that is usually summarized as "cogito ergo sum" appears first in Descartes' Meditations on First Philosophy, where he attempts to build an entire philosophical system with no prior assumptions. He reasons that since all his beliefs have been derived from potentially misleading sense data or potentially fallacious logic, he can trust nothing that he has hitherto taken to be true. That is to say, he decides to systematically doubt all that could conceivably be doubted. He discovers 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.
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http://en.wikipedia.org/wiki/Cogito_ergo_sum
-'rilla