Having introduced the argument from dreaming, Descartes now goes a step further, with the argument from deception. Descartes first considers the possibility that God might have perpetrated an elaborate hoax, giving him experiences that bear no relation whatsoever to reality:

A Deceiving God

“… firmly rooted in my mind is the long-standing opinion that there is an omnipotent God who made me the kind of creature that I am. How do I know that he has not 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? What is more, just as I consider that others sometimes go astray in cases where they think they have the most perfect knowledge, how do I know that God has not 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?”

This hypothesis, however, portrays God in terms unacceptable to theists; God, being good, would never do such a thing. Descartes therefore reformulates his concern as the evil demon hypothesis:

The Evil Demon Hypothesis

“I will suppose therefore that not God, who is supremely good and the source of truth, but rather some malicious demon of the utmost power and cunning has employed all his energies in order to deceive me. 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.”

Now, we have a sceptical concern that is far-reaching and difficult to dismiss. According to the evil demon hypothesis, our experiences of the world around us are not produced in the way that we think, by us perceiving reality. Rather, all of our experiences are produced by a powerful and evil demon, bent on our deception. This demon creates in us the impression that we inhabit a physical world, and experiences that appear to represent it. This appearance, though, is false; in fact, there is no external world.

The Brain-in-a-Vat Hypothesis

The modern counterpart of the evil demon hypothesis is the brain-in-a-vat hypothesis. In this scenario, the evil demon is replaced by a mad scientist, in whose laboratory are stored any number of brains in vats of liquid, each hooked up to a computer. Using this computer, the scientist is able to stimulate activity in the brains, causing them to have experiences as of an external world. The world that the brains experience, however, exists only in their imaginations and in that of the scientist. None of the brains’ beliefs about the external world are true. According to the brain-in-a-vat hypothesis, we are among the scientist’s collection of brains.

If either the evil demon hypothesis or the brain-in-a-vat hypothesis were true, then our experiences would be precisely as they are. There is nothing, therefore, in our experiences to disprove these hypotheses; for all we know, one of them is true. All of our evidence is consistent with our being deceived by an evil demon, or being a brain-in-a-vat.

In order to know anything about the world around us, though, we would have to know that these sceptical hypotheses are false. If I know that I am awake, then I know that I am not asleep. If I know that I am perceiving the world around me, then I know that I am not a brain-in-a-vat. As we cannot know these latter claims, neither can we know the former; we know nothing of physical reality. As Descartes confesses (though he later retracts this view):

“I have no answer to these arguments, but am finally compelled to admit that there is not one of my former beliefs about which a doubt may not properly be raised; and this is not a flippant or ill-considered conclusion, but is based on powerful and well thought-out reasons.”

These arguments have proved difficult to refute, but there are several responses to scepticism available.