Descartes' Evil Demon is a philosophical concept introduced by René Descartes in his work "Meditations on First Philosophy." It posits the idea of a powerful, deceptive being that could manipulate our perceptions and beliefs, leading us to doubt the existence of the external world and even our own thoughts. This thought experiment challenges the reliability of our senses and the certainty of knowledge. The purpose of the Evil Demon hypothesis is to explore the foundations of knowledge and skepticism. By imagining such a deceiver, Descartes aimed to strip away all beliefs that could be doubted, ultimately seeking an indubitable truth. This led to his famous conclusion, "Cogito, ergo sum" or "I think, therefore I am."