Descartes, the father of modern philosophy and the rationalist school of thought, unleashed a wave of scepticism with his Evil Demon Hypothesis (AKA “evil genius”, “brain in the vat”). His reasoning went something like this.

1. During a dream, I may take things unquestionably as true or plausible.
2. On waking from the dream, these things become doubtful, the stuff of fiction.
3. These things may concern fundamental laws of logic or nature. For example, in a dream, I may believe the proposition “2+2=5”.
4. I possess no certain way of telling whether I am awake or asleep.
5. Therefore, it is possible that I am mistaken about the nature of my “waking” life. It may be that what I take to be waking life is, in fact, a dream (or series of dreams).
6. It is possible that an evil demon exists that has the power to keep my consciousness in  a world of dreams, in which it constantly feeds me false information.
7. Therefore, if I am to establish that my waking life delivers truth to me, I must overcome the Evil Demon Hypothesis.

Descartes thought that he had overcome this hypothesis. He believed that he had shown deductively that God exists and that God is not a deceiver. From there he went on to say that the Evil Demon Hypothesis was incompatible with the existence of a truthful God: therefore, this hypothesis must be false. The attentive reader can explore these arguments in Descartes’ Meditations.

Contemporary and subsequent philosophers criticised Descartes’ work. Many have taken exception to his Trademark Argument, his ontological argument, and his cosmological argument. Others have criticised the “Cartesian Circle”: Descartes relies on what he calls “clear and distinct ideas” to prove the existence of God, and then goes on to say that God, not being a deceiver, guarantees that clear and distinct ideas convey truth to the human mind. Descartes understood these steps in a linear fashion, but his critics assert that he begged the question.

Descartes needs a non-deceiving God for his epistemology to work, once he has raised the spectre of the evil demon. Without God, he is reduced to a state of scepticism in which he can know very little indeed. As an infallibilist, Descartes understood the composition of knowledge in the following terms.

1. I believe that the proposition is true.
2. The proposition is true.
3. I cannot rationally doubt that the proposition is true.

One might argue that one cannot rationally doubt that 2+2=4, therefore, this proposition and others of the same class meet the criteria set forth by Descartes. However, if one believes that the Evil Demon Hypothesis is a rational belief to entertain, while also believing that Descartes has failed to prove the existence of God, then one could argue that the Evil Demon Hypothesis provides rational grounds for doubting that the proposition 2+2=4 is true.

We may respond in a number of ways to this problem.

1. Argue that it is irrational to entertain the Evil Demon Hypothesis.
2. Argue that God is necessarily foundational to any epistemology (and theory of language – see Quine’s work). Rather than worry about proving the existence of God, we should take God’s existence as axiomatic and concern ourselves with building upon that axiom.
3. Adopt a different definition of knowledge in place of Descartes’ infallibilist model (e.g. Nozick’s “tracking the truth” reliabilism).
4. Accept the fact that faith plays a role in epistemology (see Hume’s work on the “problem of induction”). Our aim should be to show that it is more reasonable to adopt one proposition over another (where the two are mutually exclusive).