The Latin phrase cogito ergo sum, or "I think, therefore I am" meaning "I doubt, therefore I know", is possibly the single widest-known philosophical statement, and is due to Rene Descartes.
In his book Meditations on First Philosophy, Descartes attempted to build up an entire philosophical system from scratch, with no prior assumptions. In order to begin this undertaking, he reasoned 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 led him to discover that the one thing that he could not doubt was his own existence. After all, he claimed, 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.