cogito, ergo sum
Dictum coined in 1637 by René Descartes as a first step in demonstrating the attainability of certain knowledge. It is the only statement to survive the test of his methodic doubt. The statement is indubitable, Descartes argued, because even if an all-powerful demon were to try to deceive him into thinking he exists when he does not, Descartes would have to exist in order to be deceived. Therefore, whenever he thinks, he exists. Furthermore, Descartes maintained, the statement I am (sum) expresses an immediate intuition, not the conclusion of a process of reasoning, and is thus indubitable.
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