How did the professor respond to Susan and Peter after their account of Lucy's story?

The professor in "The Lion, The Witch, and the Wardrobe" is a character that is based on partially on author C.S. Lewis himself. Therefore, it is easy to imagine Lewis himself responding to Susan and Peter when they confront him about their sister Lucy's story about her trip to a mysterious "other world" early in the narrative.
The professor calmly and quietly asks Susan and Peter about the nature of Lucy's character, and if she lies on a regular basis. After Susan and Peter both admit that Lucy is a trustworthy sibling, and acknowledge that Edmund, the brother who states that Lucy's story is a complete fabrication, the professor gives them a measured, supremely logical response.
“Logic!" said the Professor half to himself. "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 istelling 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.” - C.S. Lewis, The Lion, The Witch, and the Wardrobe.
The professor's response, while colored by his own, later-revealed experiences in Narnia, allow Susan and Peter to draw their own conclusions about Lucy's story while hinting to them that they should, perhaps, be a bit more supportive of her.

The Professor in The Lion, the Witch, and the Wardrobe has one of the most reasonable responses that could be imagined in such a situation. In reality, his calm demeanor is aided by his knowledge of Narnia from his experience there as a young boy. However, discounting this fact (since the children don't know it), he seems to have an incredibly measured, logical response.
After listening carefully and allowing the children to explain the story, he comes back with a set of conclusions: Lucy is either lying or making up a game, which seems unlike her; she is crazy, which by all accounts she is not; or she is telling the truth. In the Professor's estimation, they have to give some credence to her experience simply because there is little other room for explanation.
At the end of the discussion, he balks at the children, who argue against his conclusion. He is perturbed, questioning if they'd ever been taught logic or reasoning in school. This is humorous commentary on the society that, in Lewis's eyes, was venturing further away from reasoning and refused to believe in magical or miraculous events.