According to the ethical theory of utilitarianism, an action is right if it promises to produce better results than—or maximize the expected utility of—other action possible in the circumstances. Although there are earlier examples of utilitarian reasoning, the English philosopher Jeremy Bentham (1748–1832) gave utilitarianism its first full formulation. "Nature has placed mankind under two sovereign masters," Bentham declared, "pain and pleasure. It is for them alone to point out what we ought to do, as well as to determine what we shall do." The ethical person, then, will act to increase the amount of pleasure (or utility) in the world and decrease the amount of pain by following a single principle: promote the greatest happiness of the greatest number.

As later utilitarians discovered, this principle is not as straightforward as it seems. The very notion of happiness is problematical. Are all pleasures intrinsically equal, as Bentham...