 Professor Kirke presents three choices to the other Pevensie  children of what to think about Lucys claim about Narnia: that she  is lying, that she is mad, or that she is telling the truth.  C.S. Lewis (qv) was a vocal advocate of Christianity, and in his  book "Mere Christianity" presented three similar choices about  Jesus claim to be the Son of God: Liar, Lunatic, or Lord. This is  known as his "Trilemma" argument, and is still often used by  Christians today.