 The Cartesian Skeptic describes an alleged logically possible scenario in which our mental lives and their histories are precisely the same as what they actually are, but where the causes of the facts about our mental lives are not the kinds of events in the external world that we commonly think they are. On Descartes’ Evil Genius hypothesis, there is no physical world. Rather you are a disembodied mind, and your entire mental life, with all of its experiences, has been caused by an all-powerful, purely spiritual Evil Genius. As a result, your beliefs about the external world, such as that you have a body, or that there are planets in the solar system, are all mistaken. On the brain-in-a-vat hypothesis, a given person is a disembodied brain living in a vat of nutrients. The nerve endings of the brain are connected to a supercomputer, whose program sends electrical impulses that stimulate the brain in the same way that actual brains are stimulated when perceiving external objects. If you are a brain in a vat, suppose, your conscious experiences are and have been qualitatively indistinguishable from the experiences you have had over the entire course of your mental life. But if as the result of your computer-caused experiences you believe, say, that you have a body, then you are mistaken.