It’s like talking to a brick wall” and We’ll have to agree to disagree” are popular sayings referring to the frustrating experience of discussing issues with people who seem to be beyond the reach of argument. It’s often claimed that some people-fundamentalists or fanatics-are indeed sealed off from rational criticism. And every month new pop psychology books appear, describing the dumb ways ordinary people make decisions, as revealed by psychological experiments. The conclusion is that all or most people are fundamentally irrational. Ray Scott Percival sets out to demolish the whole notion of the closed mind and of human irrationality. There is a difference between making mistakes and being irrational. Though humans are prone to mistakes, they remain rational. In fact, making mistakes is a sign of rationality: a totally non-rational entity could not make a mistake. Rationality does not mean absence of error; it means the possibility of correcting error in the light of criticism. In this sense, all human beliefs are rational: they are all vulnerable to being abandoned when shown to be faulty. Percival agrees that people cling stubbornly to their beliefs, but he maintains that not being too ready to abandon one’s beliefs is rational.