This seems obviously false. Am I missing something?
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).
Another month, another rationality quotes thread. The rules are: