I am a teacher and rarely write for specialists alone. I have tried to avoid the dead-stick prose so beloved by journal editors. Anyone with a good education or a major in psychology should be able to read this book and the former is more important than the latter. It assumes that everyone is interested in intelligence and would like something exciting to provide a reason to learn more about it. Specialists will find that much has been omitted but will also, I hope, find something new in the argument and something worth pursuing in the research designs recommended.

A warning for everyone: there are problems that can simply be settled by evidence, for example, whether some swans are black. But there are deeper problems that pose paradoxes. Sometimes the evidence that would solve them lies in an inaccessible past. That means we have to retreat from the scientific level of explanation to the historical level where we demand only a plausibility that conforms to the known facts. I believe that my efforts to resolve the historical paradoxes we will discuss should be judged by whether someone has a more satisfactory resolution to offer. The reader should be wary throughout to distinguish the contentions I evidence from the contentions to which I lend only plausibility.