Evil Genius

In Rene Descartes' Meditations on First Philosophy, "First Meditation", Descartes hypothesizes the existence of an omnipotent, omniscient, and malevolent being who is capable of deceiving us with false beliefs. If such a being existed, how could we ever know if the beliefs we hold at any given time were true? This is Descartes’ version of the skeptical hypothesis which states that all of beliefs might be false.