People universally organize experience into categories (e.g., birds, trees, females). Standard treatments in cognitive psychology tend to focus on three basic functions: efficiency, inductive inferences, and communication. In this talk I argue that categories have a fourth basic function, to convey moral principles and norms. I review evidence for this position, by examining links between categories and norms, and links between essences and moral intuitions. Converging methods are reported, from generic language, category transgressions, beliefs about heart transplants, and beliefs about the genetic basis of criminality.