I think that C.S. Lewis means that when a person puts forth an assertion, you should ascertain the truth of falsity of the assertion by examining the assertion alone; the mental state of the person making the assertion is irrelevant.
Presumably Lewis is arguing against the genetic fallacy, or more specifically, Bulverism.
Edit: Why the downvote? My comment was fairly non-controversial (I thought).
Whether a belief is wishful thinking is inherently an assertion about the mental state of a person. It is meaningless to say that you should examine the assertion instead of the mental state, since the assertion is an assertion about the mental state.
Another month, another rationality quotes thread. The rules are: