The story begins with a philosophical discussion about how science (especially phrenology) understands human beings. It is the narrator's contention that science develops a priori. One first decides on the basic needs of humankind, then ascribes to certain organs the role of satisfying those needs. The narrator thinks this is backwards: "It would have been wiser, it would have been safer to classify . . . upon the basis of what man usually or occasionally did, and was always occasionally doing, rather than upon the basis of what we took for granted the Diety intended him to do."

One major trait scientists have overlooked is perversity. Men often do things for no better reason than that it will hurt themselves or others. The narrator then tells how he murdered a man and lived contentedly with the knowledge for many years until he was suddenly, perversely compelled to confess.