made him go wrong in mathematics ... the evil genius merely reinforces the doubt that the external world may be a dream." When Kenny says that the evil genius is simply a substitute for the deceitful God, he is not trying to establish that, therefore, the evil genius was omnipotent, instead he is challenging the view that the evil genius somehow progressed on from God and is rejecting the view that "the evil genius is to serve a more radically skeptical purpose than the hypothesis of the deceitful God." According to Janowski, the fact that the demon is not said to challenge mathematics, implies either that the evil demon is not omnipotent or that Descartes retracted Universal Doubt. Janowski notes that in the Principles of Philosophy (I, 15) Descartes states that Universal Doubt applies even to "the demonstration of mathematics", and so concludes that either Descartes' Meditation is flawed, lacking a reason for doubting mathematics, or that the charges of blasphemy were well placed, and Descartes was supposing an omnipotent evil demon. However, this is only a problem if one assumes that Descartes was withdrawing the notion of a deceitful God and replacing it with the evil demon. More recent commentators take the argument to have reached its conclusion with the deceitful God. When Descartes says, "I will suppose therefore that not God, who is supremely good and the source of truth, but rather some malicious demon..." he is not rejecting the notion of a deceitful God on the grounds that God is not a deceiver for this is something he is not entitled to rely on, because, as he says at the beginning of Meditation three, he doesn't "yet even know for sure whether there is a God at all". Instead, he is introducing an aid to the meditator who finds that, despite the arguments presented, "habitual opinions keep coming back". Kenny says, "The purpose of taking seriously the hypothesis of the evil genius is to counterbalance natural credulity and keep in mind the doubts raised by the supposition of the deceitful God." When the role of the demon is understood this way the issue of the demon's omnipotence becomes unimportant. The brain in a vat In 1968, James Cornman and Keith Lehrer suggested something they called the braino machine that "operates by influencing the brain of a subject who wears a special cap, called a "braino cap." When