The first such case I found may not be humor as much to others as it seems to be to me. It is in the very first chapter when Lewis is discussing the fear of the uncanny. He says, "Most attempts to explain the Numinous presuppose the thing to be explained- as when anthropologists derive it from fear of the dead, without explaining why dead men (assuredly the least dangerous kind of men) should have attracted this particular feeling." Lewis' side note pointing out the ridiculousness of fearing the dead just struck me as funny.
A second example in chapter two when Lewis is talking about divine omnipotence, "If you choose to say 'God can give a creature free-will and at the same time withold free-will from it' you have not suceeded in saying anything about God: meaningless combinations of words do not suddenly acquire meaning simply because we prefix to them the two other words 'God can'."
In the third chapter when Lewis is describing his own morality upon entering university, he compares himself thusly, "...of chastity, truthfulness, and self sacrifice I thought as a baboon thinks of classical music."
Upon looking these citations which I find humorous, I think that what I am trying to bring into focus isn't an outright attempt at humor but something I find humorous in the way it is said. Most of these quotes, when I read them, I was mostly amused because of how ridiculous the comparisons seemed which is probably exactly what Lewis intended. Still who doesn't think a baboon trying to understand classical music conjures a funny picture?
