‘...the whole process of human thought, what we call Reason, is ... valueless if it is the result of irrational causes. Hence every theory of the universe which makes the human mind a result of irrational causes is inadmissible, for it would be a proof that there are no such things as proofs. Which is nonsense. But Naturalism, as commonly held, is precisely a theory of this sort.’ Thus C. S. Lewis, in the first edition of Miracles. Forceful objections from Elizabeth Anscombe led Lewis to drop this passage from the second edition of his book. But even there he still clung to the general idea that while theism involves no such difficulty Naturalism somehow defeats itself: ‘...our conviction that Nature is uniform ... can be trusted only if a very different Metaphysic is true. If the deepest thing in reality, the Fact which is the source of all other facthood, is a thing in some degree like ourselves – if it is a Rational Spirit and we derive our rational spirituality from It – then indeed our conviction can be trusted. Our repugnance to disorder is derived from Nature's Creator and ours.’ Similar claims have frequently been made by Lewis's supporter Stephen Clark. In a typical passage Clark insists that ‘if we are to be able to trust our seeming capacity to understand the world, we must suppose that our minds mirror or share in the pattern and life which is the foundation of the world’. And now, in the last chapter of his Warrant and Proper Function, Alvin Plantinga has endorsed and developed what he duly acknowledges to be Lewis's idea. Plantinga does not pretend to have formulated a totally cogent argument for Naturalism's self-defeat. But he does think that he has said enough to indicate ‘a promising research program’. In what follows I scrutinize the argument which he sketches out.