The Latin cogito, ergo sum, usually translated into English as "I think, therefore I am", is the "first principle" of René Descartes's philosophy. He originally published it in French as je pense, donc je suis in his 1637 Discourse on the Method, so as to reach a wider audience than Latin would have allowed. It later appeared in Latin in his Principles of Philosophy, and a similar phrase also featured prominently in his Meditations on First Philosophy. The dictum is also sometimes referred to as the cogito. As Descartes explained in a margin note, "we cannot doubt of our existence while we doubt." In the posthumously published The Search for Truth by Natural Light, he expressed this insight as dubito, ergo sum, vel, quod idem est, cogito, ergo sum. Antoine Léonard Thomas, in a 1765 essay in honor of Descartes presented it as dubito, ergo cogito, ergo sum.

René Descartes was a French philosopher, scientist, and mathematician, widely considered a seminal figure in the emergence of modern philosophy and science. Mathematics was paramount to his method of inquiry, and he connected the previously separate fields of geometry and algebra into analytic geometry. Descartes spent much of his working life in the Dutch Republic, initially serving the Dutch States Army, and later becoming a central intellectual of the Dutch Golden Age. Although he served a Protestant state and was later counted as a deist by critics, Descartes was Roman Catholic.