Moral philosophers have often feared that what we must do is in some manner or another inconsistent with what we can do. Some have worried that morality may be literally contradictory: It may command us both to do certain acts and to refrain from doing those acts. Thus some have thought, for example, that we may be both categorically obligated not to kill and categorically obligated to kill in self-defense. If threatened with deadly force, we thus do wrong whatever we do. Others have thought that, while morality may not be contradictory, it may be conflicted: It may impose on individuals obligations that cannot be simultaneously satisfied. Sartre, for example, thought it possible that a man might be obligated both to join the Free French to defend against Naziism and to stay at home to administer to his failing mother.

My worry in this book is not that morality may issue contradictory injunctions or confront us with intrapersonal conflicts. My worry is that morality may require interpersonal combat. It may make one person's moral success turn on another's moral failure. Persons may be morally obligated to prevent others from satisfying their moral obligations and morally compelled to punish others for doing precisely what they should have done.

The fear that morality may be gladiatorial derives from the very common presupposition that those who take on certain roles within our society simultaneously take on unique obligations – obligations that do not burden those outside such roles. For example, personal roles, such as the role of a parent or a spouse, are often defined in terms of special obligations that are owed to those who are near and dear.