Cogito ergo sum. (I think, therefore I am.)

Descartes formulated this dictum during his project of “methodical doubt” in the early modern period, seeking a foundation for knowledge immune to skepticism. In the Discourse on the Method (1637), he argues that even if he doubts everything—sense experience, mathematics, and the existence of the external world—he cannot doubt that he is doubting or thinking. That act of thinking guarantees, at least in that moment, the existence of the thinker. The Latin form “Cogito ergo sum” became the famous shorthand, though Descartes first presents the point in French in 1637 and develops it more fully in the Meditations (1641).

The statement identifies self-conscious thought as an indubitable starting point for philosophy. It does not claim that thinking is the only way to exist, but that the very attempt to doubt one’s existence confirms it: if there is thinking, there must be a thinker. Descartes uses this as a foundational certainty from which to rebuild knowledge, distinguishing what can be known “clearly and distinctly” from what can be doubted. The cogito thus marks a shift toward epistemology centered on the subject and becomes a cornerstone of modern philosophy, influencing debates about consciousness, selfhood, and the limits of skepticism.

“Je pense, donc je suis.”
“Ego cogito, ergo sum.”
“Dubito, ergo cogito, ergo sum.”