I do not mean that school stories for boys and girls ought not to be written. I am only saying that they are far more liable to become ‘fantasies’ in the clinical sense than fantastic stories are. And this distinction holds for adult reading too. The dangerous fantasy is always superficially realistic. The real victim of wishful reverie does not batten on the Odyssey, The Tempest, or The Worm Ouroboros: he (or she) prefers stories about millionaires, irresistible beauties, posh hotels, palm beaches and bedroom scenes—things that really might happen, that ought to happen, that would have happened if the reader had had a fair chance. For, as I say, there are two kinds of longing. The one is an askesis, a spiritual exercise, and the other a disease.
C.S. Lewis, Other Worlds (1975), 30.