In my last post, I asked what empirical observations you would be able to make that would be different if morality existed than if it did not. Some of you may have objected that that defies the is-ought problem.
Hume said that you can't derive "ought" statements from "is" statements alone. That is, you cannot determine what is moral or how the world should be just from how the world is. For example, suppose a runaway trolley is heading toward people who are tied to the track. There is a lever, which if pulled will divert the trolley onto another track, where it will cause no harm. Those facts alone are not enough to conclude that you ought to pull the lever. You can only conclude that if you add another "ought" statement to the premise, such as, "You ought to save lives.".
If Hume is correct, then morality is not observable in any way. Which would mean either that morality does not exist, or that morality is meaningless. Either way, it would be completely pointless to talk about morality.
So why are we talking about it? For that matter, why would anyone ever talk about it? No one ever talks about unobservable flergamarms, why would they talk about unobservable morality? Why would they even come up with the idea? Maybe people are just hopelessly confused. Or maybe Hume is wrong. Maybe ought can be derived from is. The question remains: How?