Wonderful! I find it really heartening when anyone connects so well with C.S. Lewis, one of my favorite writers. I own and have read nearly all his work, including that collection. However, my paraphrase was based on something he said in his classic work of literary criticism, An Experiment in Criticism. That book has the fullest treatment of the idea that you summarize so well. It was, I think, one of his most important ideas, and he used it in various contexts in many other works.
You may be interested in the book comments of Dr. Peter Kreeft, which are based on the same philosophy: Place to share relaxing and thought-provoking videos - #194 by Conrad_Melville