Henry Sidgwick is renowned for giving classical utilitarianism its most sophisticated dress and greatly advancing substantive ethical theory. Celebrated for his clarity and cool impartiality, he developed an approach to ethical theory that profoundly shaped influential philosophers from G. E. Moore and Bertrand Russell down to R. M. Hare, John Rawls, Marcus Singer, Derek Parfit, and Peter Singer. It was Sidgwick, rather than Moore, who set the course for twentieth-century debates over the ethics and metaethics of the utilitarian view that maximizing happiness is the ultimate normative demand—that is, over such matters as the conflict between egoistic and utilitarian reasons, the distinction between total and average utility, the role of commonsense in utilitarian reasoning, the meaning of good, and the moral standing of other beings that are not human. Yet Sidgwick himself had more comprehensive intellectual, religious, and cultural concerns than most of...