But we can go further. It would be impossible to accept naturalism itself if we really and consistently believed naturalism. For naturalism is a system of thought. But for naturalism all thoughts are mere events with irrational causes. It is, to me at any rate, impossible to regard the thoughts which make up naturalism in that way, and, at the same time, to regard them as a real insight into external reality.
Perhaps this may be even more simply put in another way. Every particular thought (whether it is a judgment of fact or a judgment of value) is always and by all men discounted the moment they believe that it can be explained, without remainder, as the result of irrational causes. Whenever you know what the other man is saying is wholly due to his complexes or to a bit of bone pressing on his brain, you cease to attach any importance to it. But if naturalism were true then all thoughts whatever would be wholly the result of irrational causes. Therefore, all thoughts would be equally worthless. Therefore, naturalism is worthless. If it is true, then we can know no truths. It cuts its own throat.
— C. S. Lewis, from God in the Dock, p. 137; quoted in The Journal, A Summit Ministries Publication, June 2008