How did Descartes use the phrase “I think, therefore I am” in his theory of mind?

René Descartes introduced the dictum "Cogito, ergo sum" (I think, therefore I am) as the foundational step in his methodic doubt, a procedure designed to identify knowledge that is absolutely certain.12

By systematically rejecting all beliefs that could be doubted—including sensory experience and even basic mathematical truths—Descartes arrived at a single, indubitable fact: the act of thinking itself. He argued that even if an "evil genius" were attempting to deceive him, he must exist in order to be deceived. Thus, the cogito serves as an immediate, intuitive realization of one's own existence as a thinking subject.21

While the Discourse on Method presents this as an argument, Descartes later clarified in his Meditations that it is an immediate intuition rather than a syllogistic conclusion. This realization allowed him to establish the mind as a distinct mental substance, separate from the material body—a core tenet of his mind-body dualism. By identifying the cogito as a "clear and distinct" idea, he sought to build a foundation for further knowledge, including the existence of God and the external world.123