The lion, the witch and the wardrobe

The professor invites them into his study and listens to their story from beginning to end, without interruption. When they are finished, the Professor, to their surprise, asks them why they are so certain that Lucy's story isn't true. He asks them to consider their own past experiences. Who, he asks, is more truthful: Lucy, or Edmund? He then advises them to use logic, lamenting, "Why don't they teach logic at these schools?" Logically, he says, Lucy is either telling lies, going mad, or telling the truth. He concludes that since Lucy is not a liar, and is not going mad, she must therefore be telling the truth.