Moral relativism is the viewpoint that moral standards are not absolute, but instead emerge from social customs and other sources. The philosophical stance can be traced back at least as far as the Greek scholar Protagoras, who stated that "Man is the measure of all things;" a modern interpretation of this statement might be that things exist only in the context of the people who observe them.
Moral relativism stands in contrast to moral absolutism, which sees morals as fixed by an absolute human nature (John Rawls), or external sources such as deities (many religions) or the universe itself (as in Objectivism). Those who believe in moral absolutes often are highly critical of moral relativism; some have been known to equate it with outright immorality or amorality.
Moral relativism has sometimes been placed in contrast to ethnocentrism. Essentially, the claim is that judging members of one society by the moral standards of another is a form of ethnocentrism; some moral relativists claim that people can only be judged by the mores of their own society. (This is analogous to the stance often taken by historians, in that historical figures cannot be judged by modern standards, but only in the context of their time.) Other moral relativists argue that, as moral codes differ among societies, one can only utilize the "common ground" to judge moral matters between societies.
One consequence of this viewpoint, also known as cultural relativism, is the principle that any judgment of society as a whole is invalid: individuals are judged against the standards of their society; societies themselves have no larger context in which judgement is even meaningful. This is a source of conflict between moral relativists and moral absolutists, since a moral absolutist would argue that society as a whole can be judged for its acceptance of "immoral" practices, such as slavery. Such judgments are inconsistent with relativism, although in practice relativists often make such judgments anyway (for example, a relativist is unlikely to defend slave-owners on relativistic principles).
Another view point is the individual viewpoint, also known as emotivism, where people judge morality based on ones emotions and feelings. Universism further argues that only those individuals causing or directly affected by an action can make any judgment about the action's ultimate rightness or wrongness. Those judgments can be made on the basis of reason, experience and emotion.
The philosopher David Hume suggests principles similar to those of moral relativism in an appendix to his Enquiry Concerning the Principles of Morals (1751).