A favorite ploy of professors whose teaching on relativism is challenged in class is to ask, “If you believe in moral absolutes, what are they?” A person might offer that it’s immoral to dock the professor’s pay just because he’s Jewish, or African-American, or a woman, or approves of homosexuals (or whatever your professor’s hot-button happens to be).
Chances are, though, if you suggest a moral absolute, he’ll cite those who disagree. He may think he’s made his case, but his reasoning is circular. He’s only repeating the error you’re challenging: It doesn’t follow that if different people—or different cultures, for that matter—have moral differences that none is correct and morals are relative.
The professor’s ploy is to change the subject. He challenges you to defend your view, but you haven’t expressed a view. He, however, is making a specific claim about morality and is sidestepping the challenge. The burden of proof is on him.
Students should not be afraid to challenge their professors if they do it with grace and respect. Respond this way:
Professor, it doesn’t really matter what I believe. I’m not making the claim. You are. I may even be on your side, for all you know. I’m just asking you to prove your point.
But when I ask a fair question, you change the subject and throw it back on me. You’re the one teaching morality is relative because you think cultures have different values. I’m simply asking if that works. So, please tell me how your conclusion follows.