Going back to my example of the Comanches. Their “shared culture” countenanced murder, rape, torture and thievery, and even saw them as “good”; mostly outside the tribe but sometimes also within it.
It was a spectacularly unsuccessful “truth” notwithstanding that it was part of the “shared culture”, inasmuch as neighboring peoples eventually put an end to them precisely because of those values they held. So it could not have been a “truly” or “objectively” good course of conduct.
You’re using “good” here simply to mean “useful.” I agree that treating neighboring peoples nicely is indeed good, in this sense, if your goal is not to impel those neighboring peoples to wipe you out.
Similarly, I think it is good not to eat brains if your goal is to avoid certain illnesses.
But, as you can clearly see, “good” – when used in this way – only has a meaning in the context of individual (or societal) goals. If someone’s goal was to contract a horrible disease, then eating brains would certainly be a good thing.
Now, obviously most members of a society will probably have similar goals (survival, cooperation, owning property), so it’s not surprising that they’ll judge “good” and “bad” similarly in a great number of cases. These goals are another way of talking about values, which is precisely what I’ve been talking about in this thread.
But when moralists talk about “good” and “evil,” they’re talking about something else altogether than questions of usefuless. They’re talking about something that is intrinsically worthy of pursuing or avoiding all the time, regardless of its consequences.
I’ll give you a good example: a long, long time ago, when tribes were small, I’ll bet people had a very good reason for seeing homosexuality or masturbation as “bad” – hey, if people can get their jollies without procreating, then that’s going to have some terrible effects on the tribe! It’s “bad” in exactly the sense you mean above – i.e. it can lead to bad consequences.
But nowadays, the context is different. We no longer have to keep up our population numbers, so, correspondingly, there’s no rational reason to judge homosexuality or masturbation as “bad.”
There can only be such a reason if you assume that there are supernatural value judgments that are
unconnected to questions of usefulness. And obviously, such assumptions are magical claims for which there is no evidence.