Mo_, the burden question is interesting, I think I took a position on it too quickly. I do think the burden is on the person arguing for objective morality, because I think the empirical evidence makes a universal morality counterintuitive, and only possibly by a counterintuitive explanation (thus my subjective-on-the-bottom-objective-on-top approach). But prior to seeing that evidence, there is a strong intuition that morality is objective.
But even if we grant for that reason that the burden is initially on the person arguing against an objective morality, the existence of things like the stoning you mention should be enough to shift the burden. I might find it repulsive, but I know that to be a culturally-based stance. I know that the people doing the stoning do not think themselves immoral. How is that possible? Are they just wrong? How do we know we aren’t wrong? The vehemency of our belief that one shouldn’t stone a 13 year old for any reason does not suffice to prove that that belief is objective. While I agree that the stoners are wrong, I think the burden is shifted by that very example.
But this does get to why I agree with you on the prudence-morality connection: I think the answer to why stoning a child is wrong is that it is totally imprudent. It relies on an empirically empty metaphyics, a poor understanding of how the world works that puts a lot of big and unfounded assumptions out there that justify actions that are objectively wrong without them.
To both Anon and O_H, I don’t see why a conflict between prudence and morality is a problem. It’s just as easy to construct a conflict between morality and morality (If I have one dose of cure and two deathly sick children), or between prudence and prudence (if I can park close to the entrance or close to the exit, and not both). What does a conflict between the two do to show that they aren’t different sides of the same spectrum?
I think Only_Humean did a better job of fleshing out my claim than I did, but I would like to address this directly. As I said, there is fuzziness and errors. For all your examples, there is a well defined core about which people who speak the language will agree. People might have a variety of understandings of love or pain, but they will be nearly unanimous that a picture of war does not depict love and a picture of a blowjob does not depict pain. But the margins are very fuzzy and much more poorly established. Similarly, there’s a well established core of immoral killing, but the margins are much fuzzier. But the objectivity of the core isn’t questions, and even the degree to which a certain question is fuzzy is objective, in the sense that we can observe and predict the range of feelings and act based on that observation or prediction.
This again mirrors language: I know that a potential mate will understand certain basic facts when I tell them “I love you,” and I won’t be too let down if they don’t mean exactly the same thing when they say it to me. And we may disagree how big a promotion we’d be willing to pass up to keep our love, but neither of use will expect that the other would be willing to pay to escape love.