David Hume (1711-1776) is a colossus among philosophers, and one of his more important ideas is called Hume’s Guillotine. This is the assertion that a moral claim can’t be derived from a factual claim alone. In other words, you can’t go from an “is” statement to an “ought” statement.
It turns out the reverse is also true, which means that these two types of claim are entirely separate.
This turns out to be important because we have a good way of evaluating the truth of factual claims. Broadly, this method is called science, but because it’s a way of evaluating factual claims, it has nothing to say about purely moral claims.
Now, that’s not to say that moral decisions can’t be informed by science, where those decisions have a factual underpinning, but science has no dominion over the moral. Moral claims are arbitrary.
Take one of the classic moral dilemmas: An actor can save two people by killing one. Is the actor justified in killing the one? Science is silent on this point. It could in principle confirm that the actor can save the two by killing the one, and that inaction would result in the death of the two.
But it can’t tell us which action is right.
Mr. Spock can. The good of the many outweighs the good of the few, right? So the actor should kill the one to save the two.
But wait. If the actor kills the one, the actor is a murderer. If you think murder is bad, then you can’t agree with Mr. Spock.
It’s almost like moral dilemmas are hard and there’s no objective way to resolve them.