While benevolence, compassion, and humanity were not major virtues for the ancient philosophers, modern moral philosophers generally agree that altruism is important to morality, although they disagree about what it is, how to explain it, and what its scope should be. The nineteenth-century French theorist Auguste Comte, who first coined the term altruism, claimed that the way to end social conflict is by training people to "live for others," rather than themselves. In a popular sense, altruism means something like noble self-sacrifice. A more minimal understanding, one that many philosophers favor, is an acknowledgment that the interests of others make claims on us and limit what we may do.

Altruism made its way into moral theory when Christian philosophers added the theological virtues of faith, hope, and charity to the cardinal virtues of the Greeks. Charity, the greatest of the theological virtues, was thought to be an inner...