The Stanford Encyclopedia of Philosophy says relativism “has been, in its various guises, both one of the most popular and most reviled philosophical doctrines of our time”. I’d say of all time, at least since the ancient Greek philosopher Protagoras said “man is the measure of all things, of the things that are, that they are, and of the things that are not, that they are not”. Plato strongly disagreed.
The encyclopedia offers this by way of introduction:
Relativism, roughly put, is the view that truth and falsity, right and wrong, standards of reasoning, and procedures of justification are products of differing conventions and frameworks of assessment and that their authority is confined to the context giving rise to them. More precisely, “relativism” covers views which maintain that—at a high level of abstraction—at least some class of things have the properties they have (e.g., beautiful, morally good, … justified) not simpliciter [or simply, in themselves], but only relative to a given framework of assessment (e.g., local cultural norms, individual standards), and correspondingly, that the truth of claims attributing these properties holds only once the relevant framework of assessment is specified or supplied. Relativists characteristically insist, furthermore, that if something is only relatively so, then there can be no framework-independent vantage point from which the matter of whether the thing in question is so can be established.
So we might ask whether helium atoms have two protons. Physicists and chemists would say yes, absolutely. A simple-minded relativist might say it depends on our way of thinking or our conception of the world.
Or we might ask if human sacrifice is and has always been morally wrong. Many of us would say yes, absolutely. A relativist, not being simple-minded at all, might say it depends on what culture we’re talking about. It wasn’t morally wrong for the Aztecs 500 years ago. They thought it was necessary to stop the world from ending. It should go without saying that we’re totally against it now.
Trying to understand relativism better, I read this 500-page collection of articles on the subject. More than thirty philosophy professors and a few scholars from other disciplines weigh in. The articles were mostly interesting and not too technical. However, the only conclusions I reached are that there are many kinds of relativism, some more plausible than others, and that I need to take some time and think about which kinds, if any, are plausible to me.