Now reading The Lion, the Witch and the Wardrobe.
C.S. Lewis recycles the bogus Liar, Lunatic Lord argument from Mere Christianity on page 48. Liar, Mad or Truthful may work with real human beings that you know personally, as in the context of the story, but I am beginning to wonder if Lewis believes that this is such an irrefutable bit of logic that it can be applied widely. I can't shake the feeling that it doesn't hold up much better in the Narnia context either but I'll have to ponder that for a while.