I was looking for a quote from CS Lewis's The Last Battle, so took it as an opportunity to reread the whole book. (As is often the case, I had put two separate quotes together, neither saying exactly what I had remembered.)
In the next-to-last chapter, Lord Digory is explaining that everything from the old Narnia (or England) worth saving is found in real Narnia in Aslan's own land - that the old world was merely a reflection of the real world there. "It's all in Plato, all in Plato: bless me what do they teach them at these schools?"
In the very next paragraph, Lewis explains to the reader the feeling one gets in encountering the reflected, real, and ideal worlds, using the image of a looking-glass on a wall which reflects a real scene outside. It is the Allegory of the Cave, of course, put into modern form. I never noticed it before, though Lewis fairly clubs one on the head with the juxtaposition.