This is Lewis's trilemma argument as it appears in The Lion, The Witch, and the Wardrobe. Lucy, who has a far better reputation for truthfulness than Edmund, claims that she has been though a wardrobe into a strange land called Narnia. She claims Edmund has been there too, but Edmund denies it.
"How do you know," he asked, "that your sister's story is not true?"
"Oh, but -" began Susan, and then stopped. Anyone could see from the old man's face that he was perfectly serious. Then Susan pulled herself together and said, "But Edmund said they had only been pretending."
"That is a point," said the Professor, "which certainly deserves consideration; very careful consideration. For instance - if you will excuse me for asking the question - does your experience lead you to regard your brother or your sister as the more reliable? I mean, which is the more truthful?"
"That's just the funny thing about it, Sir," said Peter. "Up till now, I'd have said Lucy every time."
"And what do you think, my dear?" said the professor, turning to Susan.
"Well," said Susan, "in general, I'd say the same as Peter, but this couldn't be true. . ."
"That's more than I know," said the Professor, "and a charge of lying against someone whom you have always found truthful is a very serious thing; a very serious thing indeed."
"We were afraid it mightn't even be lying," said Susan, "we thought there might be something wrong with Lucy."
"Madness, you mean?" said the Professor quite cooly. "Oh, you can make your minds easy about that. One has only to look at her to see that she is not mad. . . Logic! . . .Why don't they teach logic at these schools? There are only three possibilities. Either your sister is telling lies, or she is mad, or she is telling the truth. You know she doesn't tell lies and it is obvious that she is not mad. For the moment then and unless any further evidence turns up, we must assume that she is telling the truth."
The link is to Peter Williams' discussion of the issue of incarnation.