Descartes imagined a demon devoted entirely to deceiving him about everything

To find a belief he couldn't possibly doubt, Descartes pushed skepticism to its limit: suppose a powerful, malicious demon has spent its existence feeding him a completely false picture of reality, down to the simplest facts of mathematics. Even then, he realized, the demon cannot make it false that he is thinking the thought in the first place. That one surviving certainty became the foundation of his whole philosophy.

— Rene Descartes, Meditations on First Philosophy — First Meditation, 1641