Evil demon
Overview
Imagine a powerful magical demon who controls everything you see, hear, and feel. This being could make you believe the sky is blue, that 2 + 2 = 4, or that you have a body — even if none of it is real. So how can you be sure what's real?
Notes
The evil demon, also called the Deus deceptor, malicious demon, or evil genius, is a thought experiment used by René Descartes in his Meditations on First Philosophy (1641). In the first meditation, he imagines a powerful and cunning being whose only goal is to deceive him. This demon creates the illusion of an external world so convincing that Descartes considers the possibility that everything he sees—sky, earth, colours, shapes, sounds—is an elaborate illusion. He even questions whether he has a body or senses, suspecting these beliefs might be false.
Some scholars suggest this demon might be all-powerful and capable of altering logic or mathematics. However, this view conflicts with Descartes' own stance, as he denied the demon’s omnipotence. The evil demon scenario is one of several tools Descartes uses to explore radical doubt. It is what the cogito is the answer to: whatever the demon does, there must be someone being deceived.