C.S. Lewis is easily one of the weakest philosophical writers I have ever read. I mean that both in style and substance. First, style: I don’t care to read his fiction/allegories (or watch the movies now made from them), so I’ll defer my opinion on that style to others – he probably is an excellent writer of fiction. However, one does not write philosophy as one writes fiction. And he just isn’t good at the former. Second, substance: In a chapter he wrote about miracles, he said math was tautological, then he cited basic algebra to back himself up. I rest my case on that point.
The man just wasn’t very good at the important stuff.