Many of us are familiar with the argument Puddleglum gives back to the Green Witch in Narnia.
"One word, Ma'am," he said, coming back from the fire; limping, because of the pain. "One word. All you've been saying is quite right, I shouldn't wonder. I'm a chap who always liked to know the worst and then put the best face I can on it. So I won't deny any of what you said. But there's one thing more to be said, even so. Suppose we have only dreamed, or made up, all those things--trees and grass and sun and moon and stars and Aslan himself. Suppose we have. Then all I can say is that, in that case, the made-up things seem a good deal more important than the real ones. Suppose this black pit of a kingdom of yours is the only world. Well, it strikes me as a pretty poor one. And that's a funny thing, when you come to think of it. We're just babies making up a game, if you're right. But four babies playing a game can make a play-world which licks your real world hollow. That's why I'm going to stand by the play-world. I'm on Aslan's side even if there isn't any Aslan to lead it. I'm going to live as like a Narnian as I can even if there isn't any Narnia. So, thanking you kindly for our supper, if these two gentlemen and the young lady are ready, we're leaving your court at once and setting out in the dark to spend our lives looking for Overland. Not that our lives will be very long, I should think; but that's small loss if the world's as dull a place as you say."*
Reading Adam Barkman's very thorough C. S. Lewis and Philosophy as a Way of Life (Zossima Press, 2009), I found the following gloss on it from a letter Lewis wrote a month before he died:
I suppose your philosopher son--what a family you have been privileged to bring into the world!--means the chapter in which Puddleglum puts out the fire with his foot. He must thank Anselm and Descartes for it, not me. I have simply put the 'Ontological Proof' in a form suitable for children. (Barkman, p. 91).
Those interested in a philosophical discussion of Puddleglum's argument should read Steve Lovell's "Breaking the Spell of Skepticism: Puddleglum vs. the Green Witch" in The Chronicles of Narnia and Philosophy (Open Court, 2005), pp. 41-52.