According to Peter van Inwagen, C. S. Lewis failed in his attempt to undermine naturalism with his Argument from Reason. According to van Inwagen, Lewis provides no justification for his central premise, that naturalism is inconsistent with holding beliefs for reasons. What is worse, van Inwagen argues that the main premise in Lewis's argument from reason is false. We argue that it is not false. The defender of Lewis's argument can make use of the problem of mental causal drainage, a (...) longstanding issue in philosophy of mind, to show how van Inwagen's objection fails. (shrink)