Objective morality as a proper basic belief

[Astonished]

I don't think Harm can stand alone as a moral theory let along be objective in its own right.

1. I keep seeing the statement "harm is objective". That is true only in the sense that there is harm or there isn't harm--which, by iteself, is insufficient to make moral judgments. It does nothing to address categories of harm (and their relative weight in an equation), thresholds of harm, intensity of harm, competing harms, exceptions to harm -- all of which are needed to assess moral choices--all of which are subjective.

Additionally, to assess harm, all kinds of moral value must be inferred and assigned to issues like relative (quantity) harm, comparing and grading different types of harm (physical, mental, slander, other intangible harms), intent, a higher value placed on humans, exceptions in war, punishment for crimes that can't possibly be repeated, etc. Over time and across cultures these underlying values are different, so any moral system based on harm changes along with it.

2. It does not take into account actions which may cause no harm like undiscovered adultery, instances of lying that don't have real consequences, breaking promises, etc., OR consensual harm like drug use, assisted suicide, voluntary slavery, medical testing.

3. Morality based on Harm does little to instruct us on our moral obligations to act and if you claim it can compel us to act, on what grounds?

4. It seems to hang your hat on harm alone is just a huge ball of situational ethics (the very definition of subjective) or is riding on top of another moral theory that has already established value to all the moving parts.

I partially agree with you on parts 1 and 4, in that there's nuance that is inevitably subjective, but disagree about everything else, especially 3. If what we agree on is that minimizing harm and maximizing well-being, that informs our actions because we will contemplate the ramifications of them in those terms. It's not like we're children learning how the world works, we're adults and we have functioning rational capacities. You'd have to be utterly addled to be incapable of thinking about this in critical terms and the fact that we have in-built empathy and experience with what harms us, we know how to evaluate similar situations when we see them. Yes, some situations are pretty extreme and don't necessarily have a readily evident answer but those are the vast minority but nothing prevents us from drawing upon the cumulative experiences and other objective information to come to a conclusion even if in hindsight we feel we should have acted differently, or if no one can come to a consensus and a less preferable or no action is taken. If something is so wrong with you that you don't feel compelled to act on your own moral convictions, you're either in possession of a very unsound moral foundation or are misunderstanding the entire position.