From Butterflies and Wheels (via One Good Move):
But toleration, which is often, although not always, a good thing, is not the same as relativism, which is never a good thing; and it is vital to understand the difference. In the intellectual world, toleration is the disposition to fight opinion only with opinion: in other words, to protect freedom of speech, and to confront divergence of opinion with open critical reflection rather than suppression or force. The first great champion of toleration in this sense was John Locke, and his successors included not only famous liberals such as John Stuart Mill, but men with a rather more direct impact on human affairs, such as Thomas Jefferson. Toleration entered political life with the Enlightenment. It is a characteristically secular virtue: there has never been and never will be a theocracy that can wholeheartedly applaud it. For the religious mind, many sayings are not to be assessed at the bar of truth or falsity, but at that of blasphemy, and to hold that a person blasphemes is to hold that that person’s sayings at least, and the person for preference, must be suppressed.
Toleration gives us the dictum attributed to Voltaire, that I disapprove of what you say, but I will defend to the death your right to say it. Relativism, by contrast, chips away at our right to disapprove of what anybody says. Relativism names a loose cluster of attitudes, but the central message is that there are no asymmetries of reason and knowledge, objectivity and truth. There are two relativistic mantras: “Who is to say?” (who is to say which opinion is better?) and “That’s just your opinion” (your opinion is on all fours with any other). There are only different views, each true “for” those who hold them. Relativism in this sense goes beyond counselling that we must try to understand those whose opinions are different. It is not only that we must try to understand them, but also that we must recognize a symmetry of standing. Their opinions “deserve the same respect” as our own. So, at the limit, we may have western values, but they have others; we have a western view of the universe, they have theirs; we have western science, they have traditional science; and so on.
There have been many philosophical attempts to refute relativism, beginning perhaps with Plato’s encounter with sophists such as Gorgias or opponents such as Theodorus in the Theaetetus. Theodorus defends Protagoras’s doctrine that Man is the Measure of All Things, which Socrates takes to imply relativism. The central tactic Socrates uses is to query whether the relativistic doctrine applies to itself. If it does not, then it seems that there is at least one non-relative, absolute truth. If it does, then indeed relativism may be true for Protagoras, but remains untrue for Socrates and the rest of us who agree with him.