The evil demon, also known as Deus deceptor, malicious demon, and evil genius, is an epistemological concept that features prominently in Cartesian philosophy. It is one of several methods of systematic doubt that Descartes employs in the Meditations. and Alan Musgrave, make no distinction between the deceiving God and evil demon arguments and regard anything said about the deceiving God as being equivalent to saying something about the evil demon.
Other writers acknowledge that Descartes makes mention of both but then claim they are 'epistemologically equivalent'. Anthony Kenny says, "the two hypotheses do not differ in any respect of epistemological importance... The content of the two hypotheses is the same..." Lex Newman says, "Descartes' official position is that the Evil Genius Doubt is merely one among multiple hypotheses that can motivate the more general hyperbolic doubt... Even so, I regularly speak in terms of the evil genius... as a kind of mnemonic for the more general doubt about our cognitive nature."
If they are epistemologically equivalent, then the question arises as to why Descartes temporarily shifted from the deceiving God to the evil demon. It is tempting to think it is because there is a relevant theological difference. In Meditation Three, Descartes is going to establish not only that there is a God but that God is not a deceiver. When Descartes first introduces the evil demon he says, "I will suppose therefore that not God, who is supremely good and the source of truth, but rather some malicious demon, had employed his whole energies in deceiving me." Kenny says, "The hypothesis of the evil genius is substituted for that of the deceitful God simply because it is less offensive and less patently incoherent." He adds, "the 'demon-argument' is not an argument at all. Descartes does not need another argument at this stage: the dream argument has already shown the unreliability of the senses and the deceiver-God argument the uncertainty of mathematics. For one thing, the demon does not even touch mathematics or geometry. Why should he? He is evoked by Descartes to cure his inordinate attachment to the senses; he does not complain (and would not) of a similar attachment to mathematics or geometry." adding, "It seems likely that he chose to call his hypothetical deceiver a "malicious demon" in order to avoid having the meditator concentrate extensively on the thought that God could be a deceiver, a proposition he considered false and one he intended to refute later." She adds in the accompanying footnote that, even if one has to concede that the text doesn't reveal any sharp distinction between the power hypothetically ascribed to the 'malignant spirit' and that genuinely attributable to God, "Gouhier's observation is essentially accurate, and useful in understanding the rhetoric and organization of the first three Meditations. It may also have some deeper significance, because of the association ... of the possibility of deception in mathematics with the doctrine of the creation of the eternal truths." The braino argument was intended to show that, even if it is sometimes possible for a person to tell if they are hallucinating, it is not possible for them to know that they are not hallucinating. If the braino is operated by an evil being, whom Cornman and Lehrer call Dr. O, then it would be possible for Dr. O to create in a person experiences that are identical to the ones of the supposed real world. If that were the case, then the experiences thus created would not constitute knowledge, for the source of those experiences would be the machine and not the world. However, since the fake and the real would be hypothetically indistinguishable, it follows that the current supposed experiences of the real world are also insufficient to generate knowledge. This proof by contradiction that the real world cannot generate knowledge, and thus that the fake imitation world cannot generate knowledge either, is one proposed way of disproving the Brain in a Vat argument.
In 1973, in the introduction to his book Thought, Gilbert Harman said, "it might be suggested that you have not the slightest reason to believe that you are in the surroundings you suppose you are in ... various hypotheses could explain how things look and feel. You might be sound asleep and dreaming or a playful brain surgeon might be giving you these experiences by stimulating your cortex in a special way. You might really be stretched out on a table in his laboratory with wires running into your head from a large computer. Perhaps you have always been on that table. Perhaps you are quite a different person from what you seem..."
Such scenarios had been used many times in science fiction but in philosophy it is now routine to refer to being like a 'brain in a vat' after Hilary Putnam produced an argument which, ironically, purported to show that "the supposition that we are actually brains in a vat, although it violates no physical law, and is perfectly consistent with everything we have experienced, cannot possibly be true. It cannot possibly be true, because it is, in a certain way, self-refuting."
Putnam's argument notwithstanding, the brain in a vat scenario is usually presented as a sceptical argument and in many ways equivalent to Descartes' deceiving God and evil demon.
One crucial difference that prevents such scenarios being a direct substitute for the deceiving God and evil demon is that they generally presuppose that we have heads or bodies whereas it is important for Descartes to argue that he can doubt the existence of his body and that he can only be sure he is a 'thinking thing'.