Into the Wardrobe

I remember reading in one of C. S. Lewis's works a passage about how people tend to think of the 13th century when they think of the Middle Ages, even though by the 13th century the Middle Ages were already starting to ossify, as exemplified by Thomas Aquinas and Scholasticism, which were rather stiff and immovable like a fully-armored knight (who was rather like a lobster in all that armor).

Or something like that. I could be wrong on any number of details, but that is the general thrust of the passage.

Does anyone recognize what I'm talking about? I've been looking and looking, and so far I've had no luck finding the passage. I'm starting to think that the passage exists only in my imagination.

I know this isn't really helpful, but I don't think it is your imagination. I seem to remember something of the sort (don't remember the lobster bit, though). Have you looked in The Discarded Image?