According to a philosophical commonplace, Aristotle defined human beings as rational animals. When one takes a closer look at the surviving texts, however, it is surprisingly hard to find such a definition. Of course, Aristotle repeatedly stresses that he regards rationality as the crucial differentiating characteristic of human beings, but he nowhere defines the essence of what it is to be human in these terms. What is more, Aristotle’s abundant remarks about human nature are scattered throughout his texts, and he offers no systematic treatise on human beings.