What IS good, and how do we determine it?

[Esquilax]

Some atheists who believe that objective moral right and wrong really do exist, a position also called moral realism, say they simply know that these moral truths exist, and that’s good enough for them. For example, Walter Sinnott-Armstrong answers the question, “Why is it wrong to cause harm without a good reason?” by saying, “It just is, don’t you agree?”

I wouldn't say that. I would, however, say that moral systems, by their nature, must privilege the continuation of conscious entities; without conscious beings to act and rationalize within a moral continuum, there can be no morality at all. Inanimate objects cause no actions, and thus can have no actions with moral weight. For morality to be self sustaining in any way, it must concern itself, in some measure, with the survival and flourishing of moral actors. If it doesn't, it dies out, due to a form of natural selection for ideas.

It isn't a coincidence, for example, that moral systems that suggest that suicide is morally optimal do not exist, at least for long.