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.
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.
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.
I don’t know about that. Merriam Webster defines wishful thinking as:
So if my calculations are accurate, per Merriam Webster’s definition, I have not engaged in wishful thinking.