How did the Professor respond to their account of Lucy’s story?

Peter and Susan are concerned about Lucy as well as Edmund's cruel treatment of her. They both go to seek the Professor's advice. 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?" You can find this in chapter five of the book.