For most twentieth-century philosophers, intent on dividing philosophy into discrete subdivisions, the problem of evil was a matter for philosophical theology, or—more rarely—for ethics and moral psychology. The theological question is as easy to state as it is hard to answer: How is a world full of evil and suffering compatible with the existence of an omnipotent, benevolent creator? The ethical question is altogether different: How can rational beings commit evil acts? The first question has preoccupied theists since The Book of Job; the difficulty of finding a satisfactory answer has served many as a reason for rejecting theism. The second question has been answered in some religious traditions by the appeal to original sin, but in recent years more it has often been viewed as outside the focus of traditional philosophy. The history of modern philosophy reveals that the problems are related, and part of...