But in all such claims you beg the question of your own view. This is the argument you always avoid:
- All moral claims are X
- Not all ought-claims are X
- Therefore, not all ought-claims are moral claims
Yet you always refuse to argue your position, instead begging the question of (3). If you can’t give anything resembling a definition of “moral”, then it is not philosophically permissible to make all of your arguments on the basis of the definition of morality. If you have no way to justify (3) then you can’t keep appealing to it blindly.
In moral philosophy (3) is often affirmed on the basis of the idea that what is moral is that which involves interpersonal action (i.e. it has to do with actions that affect others). But this does not amount to your sui generis view.
But isn’t this all imported? Doesn’t it all go back to your misreading of Moore? Which philosophers actually maintain your view that “moral” is something sui generis and undefinable? If you are idiosyncratic in that usage then it seems to me that you are merely importing your idiosyncratic view into other ill-defined terms like “moral nihilism.”
As I said to @Count_Timothy_von_Icarus, it’s not at all clear whether moral relativism is a species of “moral nihilism” (where the latter term seems to have no well-defined meaning in the literature):