In order to describe the logic of morality, "contractualist" philosophers have studied how individuals behave when they choose to follow their moral intuitions. These individuals, contractualists note, often act as if they have bargained and thus reached an agreement with others about how to distribute the benefits and burdens of mutual cooperation. Using this observation, such philosophers argue that the purpose of morality is to maximize the benefits of human interaction. The resulting "contract" analogy is both insightful and puzzling. On one hand, it captures the pattern of moral...