Many philosophers believe there is a principle of practical reason that directs the rational agent to maximize the satisfaction of his own desires and interests. I will call this “the egoistic principle,” and the person who believes in it an “egoist.” Some philosophers believe that conformity to the egoistic principle is equivalent to the pursuit of happiness, or – if these are different – to the pursuit of the individual's own good. In the social sciences, especially economics, it is widely believed that some form of the egoistic principle is both normative and descriptive: that is, that it tells us not only how we should act, but also how, at least in clear-headed moments, we do act. Philosophers who endorse this view sometimes take the egoistic principle to be definitive of practical rationality, and therefore suppose that the way to show that we have “reason to be moral” is to show that conformity to moral requirements will somehow maximize the satisfaction of our own desires and interests.

This is not, of course, how the rationality of morality has been understood in either the Kantian or the rationalist tradition. Both Kant and Sidgwick, for instance, claimed that the moral principle is a principle of reason in its own right. But they also accepted the idea that something like the egoistic principle is a normative rational principle. For Sidgwick, the egoistic principle is a rival to the moral principle of utility.