This is a book about the role that psychological impairment should play in a theory of criminal liability. Criminal guilt in the Anglo-American legal tradition requires both that the defendant committed some proscribed act and did so with intent, knowledge, or recklessness. The second requirement corresponds to the intuitive idea that people should not be punished for something they did not do "on purpose" or if they "did not realize what they were doing." Although intuitive, this underlying idea can be highly controversial in practice, especially in cases involving the insanity defense. This...