Moral relativism is the belief that objective truth either doesn’t exist or cannot be known. This, of course, applies only to truth about matters of morality, not science or history since those truths are pretty easily proven and demonstrated. There is no contradiction then for a moral relativist to say that gravity is true; gravity isn’t a matter of morality.
Moral relativism comes from the requirement for a legitimate authority to hand down the truth. For example, if I say the Earth revolves around the sun, as opposed to the other way around, that’s only my opinion and can easily be dismissed. However, if a professional astronomer says that, and virtually every other astronomer agrees, now we have an authority. Now we can say that the Earth truly does revolve around the sun. It’s not longer my perception; it is established as truth.
If there is not such authority available, then we are only left with our perceptions of that truth. We are only left with our gut feelings or hunches fed by our life experiences. An example may be “do aliens exist?” We have no authoritative answer so we’re each left to pretty much guess. You may say “yes” because in an infinite universe there must be life somewhere or you may say “no” because we would have found life by now.
Who’s right? Who knows? We have no authority to decide the truth; we’re stuck in a bit of a stalemate. That stalemate is called relativism.
The thing with morality is that the only authentic authority is God. When we say something is moral or immoral we are saying simply that it is right or wrong. More specifically that thing either conforms to, or goes against, God’s will. In this sense, God is a requirement for there to be moral truth.
Moral relativism denies that God exists or that he can communicate His will to us. In other words, we have no higher authority to appeal to. So if you say abortion is immoral and I say it’s perfectly moral, how do we know who’s right? Science can’t tell us, science can only study matter, not tell us the will of God, whom you may deny exists. So who’s right? We can’t know; we are at a stalemate.
This is why people say they are “personally opposed to abortion but can’t push their will on other people.” Do you see how everything is possessed by that person? “personally” opposed to abortion. “my” will. Everything is relative to the person speaking; not to an objective truth given to us by God. This is moral relativism at its finest (worst?).
In other posts we’ll talk about some of the difficulties with moral relativism; especially when it comes to tolerance and its other glossy side-effects. For today, I just want to give an idea of what it is.