The idea of rationality is central to the explanation of human behaviour. Only on the assumption that people are at least typically rational can we attribute beliefs, motives, and desires to people – the assumption of rationality provides the “glue” that holds disparate beliefs, desires, and actions together in a coherent system. Imagine explaining a routine event, such a motorist slowing down when approaching a pedestrian crossing. The motorist, we might suggest, noticed that some people were near the crossing, believed that they were about to cross, wanted to avoid colliding with them, believed that collision might occur if the car continued at its current speed, and so on. The goal of such explanation is to provide a rationale for a person’s behaviour, explaining how they understood and acted upon the world, from their point of view. But constructing a rationale for a piece of behaviour will only provide an explanation for it if we assume that people are sensitive to such rationales; that is, unless people exhibit rationality.