Why did Descartes consider “I think therefore I am” indubitable?

René Descartes considered the dictum "cogito, ergo sum" (I think, therefore I am) to be indubitable because it survives the process of methodic doubt, a systematic approach to finding certainty by setting aside any belief that could possibly be false.12

Descartes argued that even if an all-powerful "evil genius" or demon were to deceive him about the nature of the world or his own sensations, the act of being deceived requires that he exist to be the subject of that deception. If he is thinking—even if those thoughts are false—he must necessarily exist to perform the act of thinking.134

While the phrasing "therefore" in the Discourse on Method might suggest a logical deduction, Descartes clarified in his Meditations on First Philosophy that the statement is an immediate intuition. It is a direct, unmediated mental apprehension of his own existence that cannot be doubted, serving as the foundational certainty upon which he attempted to rebuild his knowledge of the world.134