Professor Quarles is the most lovable, the most learned, the most unpoliceman-like detective we have yet met. As a matter of fact he is not a detective at all, but a scholar, specialising in philosophy and keenly interested in the human motives behind every act. Criminal cases interest him only when it is necessary to find this motive. He confesses that he does not make his theory from facts. He finds his theory of motives first and makes the facts fit it. It is an interesting method, and as after all human nature, even if warped and twisted out of humanity by circumstances, is at the back of every crime, the professor is not so far wrong when he insists that philosophy and imagination are good guides for detective work.