The evil demon, also known as Deus deceptor, malicious demon, and evil genius, is an epistemological concept that features prominently in Cartesian philosophy. In the first of his 1641 Meditations on First Philosophy, Descartes imagines that a malevolent God or an evil demon, of "utmost power and cunning has employed all his energies in order to deceive me". This malevolent God or evil demon is imagined to present a complete illusion of an external world, so that Descartes can say, "I shall think that the sky, the air, the earth, colours, shapes, sounds and all external things are merely the delusions of dreams which he has devised to ensnare my judgement. I shall consider myself as not having hands or eyes, or flesh, or blood or senses, but as falsely believing that I have all these things."

Some Cartesian scholars opine that the malevolent God or evil demon is also omnipotent, and thus capable of altering mathematics and the fundamentals of logic, though omnipotence of the malevolent God or evil demon would be contrary to Descartes' hypothesis, as he rebuked accusations of the evil demon having omnipotence. It is one of several methods of systematic doubt that Descartes employs in the Meditations.

Prior to the Meditations proper, Descartes gives a synopsis of each Meditation and says of Meditation One that "reasons are provided which give us possible grounds for doubt about all things, especially material things" and that whilst the usefulness of such extensive doubt may not be immediately apparent, "its greatest benefit lies in

The eventual result of this doubt is to

Descartes offers some standard reasons for doubting the reliability of the senses culminating in the dream argument and then extends this with the deceiving God argument. Descartes refers to "the long-standing opinion that there is an omnipotent God who made me the kind of creature that I am" and suggests that this God may have "brought it about that there is no earth, no sky, no extended thing, no shape, no size, no place, while at the same time ensuring that all these things appear to me to exist just as they do now". Furthermore, this God may have "brought it about that I too go wrong every time I add two and three or count the sides of a square, or in some even simpler matter, if that is imaginable".

After the deceiving God argument Descartes concludes that he is "compelled to admit that there is not one of my former beliefs about which a doubt may not properly be raised". ^{[citation needed]}

It is only after arriving at this conclusion that Descartes introduces the evil demon.

Although Descartes has provided arguments for doubting all his former beliefs he notes that "my habitual opinions keep coming back". It is to deal with this problem that Descartes decides he must do more than just acknowledge that the beliefs are open to doubt and must deceive himself, "by pretending for a time that these former opinions are utterly false and imaginary" and that he shall do this "until the weight of preconceived opinion is counter-balanced and the distorting influence of habit no longer prevents my judgement from perceiving things correctly".