Cogito ergo sum (Latin ‘I think, therefore I am’), the starting point of Descartes’s system of knowledge. In his Discourse on the Method (1637), he observes that the proposition ‘I am thinking, therefore I exist’ (je pense, donc je suis) is ‘so firm and sure that the most extravagant suppositions of the skeptics were incapable of shaking it.’ The celebrated phrase, in its better-known Latin version, also occurs in the Principles of Philosophy (1644), but is not to be found in the Meditations (1641), though the latter contains the fullest statement of the reasoning behind Descartes’s certainty of his own existence. See also DESCARTE. J.C.O.