One of the fundamental assumptions in economics and decision theory is that individuals—or “agents,” as they are often called—are rational. This means, in particular, that their preferences obey certain intuitive properties, called axioms. For example, if an agent prefers A to B (we can write A > B) and B to C (B > C), it is natural—and rational—to assume that they also prefer A to C (A > C): this is the axiom of transitivity. But what is true at the individual level suddenly becomes much less...