Jason Pratt, of the Christian CADRE, explains Lewis's important point about literal and metaphorical language in the chapter of Miracles entitled Horrid Red Things. One of my favorite C. S. Lewis points.
Lewis uses the example of a little girl who thinks that poison, in any given substance, is "horrid red things". She really believes that if she separated the poison out of 'poisonous' solids and liquids, the poison would really look like horrid red things. But an adult who attempted to refute her claim that lye is poisonous by correcting her false belief about what 'poison' looks like, would still be in for a nasty shock if he drank it! Indeed, with a little investigation he might have discovered that she did not believe lye poisonous because it contained horrid red things (which she knows she cannot see in the lye), but because her mother (who may have sufficiently accurate reasons for saying so) has told her the lye is poisonous and she trusts her mother. She thinks the red things are in the lye, not because she can see them, but because she already believes the lye is poisonous; therefore it must (as far as she is concerned) have those horrid red things in it somewhere. Her imagery turns out to be, upon fair examination, ultimately of little importance to the issue at hand: whether lye really is poisonous. If she was corrected about the nature of poison, it would probably not (nor should not) affect her belief about the toxicity of lye. She would know more, but she would not necessarily be refuted in her core belief.