Another look at that controversial paragraph in Lewis
Talking about the evolutionary explanation of reason, Lewis wrote:
But the very attempt is absurd. This is best seen if we consider the humblest and almost the most despairing form in which it could be made. The Naturalist might say, 'Well, perhaps we cannot exactly see--not yet--how natural selection would turn sub-rational mental behaviour into inferences that reach truth. But we are certain that this in fact has happened. For natural selection is bound to preserve and increase useful behaviour. And we also find that our habits of inference are in fact useful. And if they are useful they must reach truth'. But notice what we are doing. Inference itself is on trial: that is, the Naturalist has given an account of what we thought to be our inferences which suggests that they are not real insights at all. We, and he, want to be reassured. And the reassurance turns out to be one more inference (if useful, then true)--as if this inference were not, once we accept his evolutionary picture,  under the same suspicion as all the rest. If the value of our reasoning is in doubt, you cannot try to establish it by reasoning. If, as I said above, a proof that there are no proofs is nonsensical, so is a proof that there are proofs. Reason is our starting point. There can be no question either of attacking or defending it. If by treating it as a mere phenomenon you put yourself outside it, there is then no way, except by begging the question, of getting inside again.
Look at the last sentence. It seems that this is a Thomas Nagel point that thought cannot be understood from the outside, that apart from the perspective of "what it is like to be" a reasoner. (See ch. 2 of The Last Word: Why Can't Understand Thought from the Outside). The doubt arises because in giving a naturalistic account we invariably look at it as opposed to along it; we view it from a third person rather than a first-person perspective.
Read in this way, I think this makes Lewis's point to be something that isn't just a crass error.