Ought indicates duty or correctness. So for sure you have a duty not to murder. That duty is imposed by society. The evidence of this can be observed in many forms of indoctrination as well as in the operation of our legal system. This is as evident as the shape of the earth.
Why should we fulfill our duties?
As for correctness, there's what's deemed proper, which is defined socially. Murder is objectively not proper.
One shouldn't murder because murder isn't proper, okay. Why should I be proper?
Then there's correctness as in being true. So the question becomes: "is it true to not murder?" This is a nonsense question. In this respect the statement "one ought not murder" is neither true nor false but instead is absurd.
Eh, I'm glad we're getting closer to agreeing, I guess. But this reasoning is bizarre. "Is it true that I shouldn't murder?" is the question. If you really need another word other than "ought" then "Do I have a duty to not murder?" I guess works, but you answered that in the affirmative already.
Going through this little exercise just reinforces my sense that "ought" is only about the emotional sanctions or rewards that biological and social conditioning places on our actions. It makes no sense to ask about ought in terms of anything else.
Eh, I can wrap my head around it. I mean, it's wrong, but it makes sense. Basically, imagine for every choice there are two potential realities. Ought implies a future event that hasn't been realized yet. One of those realities is the one that's supposed to happen (or the ideal) and one of those realities is not supposed to happen (the wrong one). All is right with the world, and all that. What makes a reality ideal, though? That's all about personal feelings. So lemme dream up how this conversation will go based on the reasons you've given me so far.
D-Wood: You shouldn't murder.
Orel: Why shouldn't I murder?
D-Wood: Because you have a duty imposed by society to not murder.
Orel: Why should I fulfill my duties?
D-Wood: Because society will penalize you if you don't.
Orel: Why shouldn't I receive penalties?
D-Wood: Because you won't like the penalties.
Orel: Why should I only get what I like?
And so on. Or we'll hit the bottom and something will just seem so obvious that you think I'm bonkers for asking "Why?" but they
all have to be justified for the top to be justified. Every single step hinges on the next step being true. You can try it if you want, maybe the questions and answers will be a little different. But there's no brute fact of an "ought" so it can't be justified, ultimately.
It's only absurd when you stop to think about it, though. It comes naturally. I agree the idea of a true "ought" is absurd, but we all do it all the time. Humans have a hard time distinguishing between our perceptions and reality. I mean, look at how we describe subjective things. "This chocolate ice cream is delicious". Really? Deliciousness is a property of chocolate ice cream? Of course not. Deliciousness describes the experience I have with chocolate ice cream that happens entirely inside my brain. Not in the ice cream. Not on my tongue.
When we reason about morality, we're reasoning about how to make the world a place that we'll be happy to live in it. We all just take for granted the premise that "I ought to be happy". We assume it's a goal we should work towards. It can't really be true in any real sense, but so what? The opposite isn't true either.