"1. No belief is rationally inferred if it can be fully explained in terms of nonrational causes.
Support: Reasoning requires insight into logical relations. A process of reasoning (P therefore Q) is rational only if the reasoner sees that Q follows from, or is supported by, P, and accepts Q on that basis. Thus, reasoning is trustworthy (or "valid", as Lewis sometimes says) only if it involves a special kind of causality, namely, rational insight into logical implication or evidential support. If a bit of reasoning can be fully explained by nonrational causes, such as fibers firing in the brain or a bump on the head, then the reasoning is not reliable, and cannot yield knowledge.
2. If naturalism is true, then all beliefs can be fully explained in terms of nonrational causes.
Support: Naturalism holds that nature is all that exists, and that all events in nature can in principle be explained without invoking supernatural or other nonnatural causes. Standardly, naturalists claim that all events must have physical causes, and that human thoughts can ultimately be explained in terms of material causes or physical events (such as neurochemical events in the brain) that are nonrational.
3. Therefore, if naturalism is true, then no belief is rationally inferred (from 1 and 2).
4. We have good reason to accept naturalism only if it can be rationally inferred from good evidence.
5. Therefore, there is not, and cannot be, good reason to accept naturalism"