Earlier quoted context omitted.
> Good is purely subjective. This is a substantial claim, and requires substantial evidence. Claiming that the Good (meaning the metaethical object) is objective is also a substantial claim, but observe that it's not necessary to be a moral realist to think that things can be, for all practical purposes, bad : in an intersubjective sense, a thing is bad if everybody around you agrees that it's bad. They can all be wr…
To say good isn't subjective is to say there is some part of the universe at some fundamental level that defines good. That's the claim that requires evidence. Show me the "good particle" and then we'll talk. Until then I can only rationally assume something doesn't exist until proven otherwise.
Some of the more interesting (to me) arguments of the realists/naturalists (the justification question rather than the ontological question above), consider there to be emergent properties from the nature of interaction between agents. Basically game-theory. Presume we all were psychopaths, but intelligent self-maximizing agents and not the irrational kind. What behaviors would maximize achieving our ends? What behaviors would we need to hold others accountable to support our ends? You can take this thought experiment pretty far to get an "emergent" moral system with first-order properties similar to those we see in real societies. I find this insufficient though, you still need some base value system. Even the psychopath example presumes a set of ends for individuals like "continued survival" and so boils down to something like a form "utilitarianism."