This is because such an account of what it means to hold a belief rationally allows for no distinction between reasons for holding a belief and rationalisations of that belief. Suppose that chancy Charlie decides what to believe on a certain subject through a game of chance (by associating the various positions that might be held on the subject with the different possible outcomes of the game). Even though Charlie, being an intelligent and creative fellow, can produce formidable arguments for the position he adopts he surely does not hold that belief rationally. The problem here is that the reasons that Charlie offers in support of his belief are not really his reasons. To count as his reasons those reasons must at least partially explain why Charlie believes as he does. That is to say, the reasons are to justify Charlie’s belief those reasons must be part of what brings it about that Charlie believes as he does. It may have been something like this concern that Lewis was voicing in the central passage, quoted above. What does it take for a person’s reasons to be a part of what brings it about that they believe as they do? It seems to me to take, at least, the truth of both P4 and P5, which to remind the reader, ran as follows.
P4) The apprehension of logical laws plays an explanatory role in the acceptance of the conclusion of the argument as true.P5) The state of accepting the truth of a proposition plays a crucial explanatory role in the production of other beliefs, and propositional content is relevant to the playing of this role.