Ethical objectivists hold that there is one and only one correct system of moral beliefs. From such a standpoint it follows that conflicting basic moral principles cannot both be true and that the only moral principles which are binding on rational human agents are those described by the single true morality. However sincerely they may be held, all other moral principles are incorrect. Objectivism is an influential tradition, covering most of the rationalist and naturalist standpoints which have dominated nineteenth and twentieth century moral philosophy: there is widespread agreement amongst relativists themselves that objectivism is firmly rooted in common sense.

Moral relativism is an important alternative to this view. Relativists challenge objectivism by drawing attention to the extent of moral diversity between different cultures; to the variation in morals within a given society at different historical epochs; and to the existence of a remarkable degree of moral disagreement within cultures at a single period of time. In the light of such diversity relativists argue that the objectivists' belief in the existence of a single true morality is a product of human ethno-centrism and, invoking Protagoras, suggest the more modest thesis that the moral opinions ‘of each and every one are right’ (Theaetetus 162a, Plato 1961). Traditional moral relativism therefore, normally involves the theses that different societies hold incompatible basic moral principles, that each of these incompatible principles is in some sense correct, that morality has its foundations in varying human affective dispositions and that, as a consequence, there is no single true morality.