“I think, therefore I am” is the popularized formulation of Descartes’ famous cogito ergo sum (hereafter, “cogito”). The cogito's epistemological significance is supposed to derive from its status as an utterly self-evident truth – “the first and most certain of all to occur to anyone who philosophizes in an orderly way” (AT VIIIA 7, CSM I 195). “Orderly” philosophizing involves a program of methodic doubt – doubt resistance, or indubitability, being the central criterion of knowledge. Some texts express the cogito's underlying point in terms of doubt (itself a form of thinking): “It is not possible for us to doubt that we exist while we are doubting; and this is the first thing we come to know” (AT VIIIA 6f, CSM I 194). Barry Stroud (2010, 518) remarks that the cogito “is certainly among the most important and longest-lasting ingredients of Descartes's legacy.” Though Descartes’ treatment of the cogito is history's most famous, it is arguably not the first. Augustine of Hippo presented a remarkably similar version of his own: “If I am mistaken, I exist” (Si fallor, sum). The differences in formulation are not insignificant. Lively debates persist concerning Descartes’ own formulation.

The most serious debate about formulation concerns inference. Versions of the cogito appear in each of Descartes’ three main published philosophical works. The “canonical” formulation (as I shall call it) includes an explicit inference – “I am thinking, therefore [ergo] I exist.” This version appears in two of the works: the Discourse (1637) (je pense, donc je suis), and the Principles (1644) (ego cogito, ergo sum). However, Descartes’ masterpiece, the Meditations (1641), presents a rather different formulation. The formula there occurs early in the Second Meditation in the context of an effort to find an indubitable truth: “So after considering everything very thoroughly, I must finally conclude that this proposition, I am, I exist [Ego sum, ego existo], is necessarily true whenever it is put forward by me or conceived in my mind” (AT VII 25, CSM II 17). Which of these represents Descartes’ official formulation? What follows are three main interpretive options.

One option is a noninferential interpretation. The most influential account comes from Jaakko Hintikka, who argues that the cogito should be understood as a performative utterance.