What role did the evil genius thought experiment play in Descartes’s philosophy?

In Cartesian philosophy, the evil genius thought experiment serves as a tool for methodic doubt, a systematic procedure used by René Descartes to identify indubitable truths by rejecting any belief that could possibly be false.12

Descartes posited that an all-powerful, cunning "evil genius" could be deceiving him about everything, including sensory experiences, the existence of the external world, and even simple mathematical truths. By imagining this scenario, Descartes pushed skepticism to its absolute limit, effectively setting aside all knowledge derived from tradition, senses, and reason.12

The experiment plays a pivotal role because it clears the path to the only remaining certainty: the cogito. Descartes realized that even if an evil genius were deceiving him, he must exist to be deceived, leading to the famous conclusion, "I think, therefore I am". This intuition provides the foundational "clear and distinct" idea upon which he attempted to rebuild his metaphysical system, eventually using the existence of a non-deceiving God to validate other knowledge.1324