A masterful follow-up to The Language of Morals. This is where Hare's universal prescriptivism develops into a full-blown normative theory. His main thesis is that the logical properties of moral language commit us to a form of preference-utilitarianism. He argues that, in judging that an action is right or wrong, I must imagine myself in the place of all those possibly affected by that action. Consequently, I must weigh the interests of all those concerned to see what is in the overall interest impartially construed. Still one of the most ingenious defenses of utilitarianism, even though it has in recent years been more or less superseded by philosophers of a more realist bent.
Hare's continuing development of the prescriptivist model who started in "Language Of Morals" sees him grappling with the realities of moral argument and trying to deal (not too well) with the charge that he is simply preloading the subject with his own liberal values whilst presented them as impartial logical truths.