Cogito ergo sum

The Latin phrase Cogito ergo sum, or "I think, therefore I am", comes from the philosophy of Rene Descartes.

In his book Discourse on Method, Descartes attempted to build up an entire philosophical system from scratch, with no prior assumptions. In order to begin this undertaking, he reasoned that he would trust nothing he had assumed to be true, and began with the decision to doubt everything that he thought he knew to be true. However, this led him to conclude that the one thing that a doubter could not doubt was his or her own existence. Thus the formulation, "I think, therefore I am", was the starting point of his philosophy.

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