He doesn’t have a proof that it is, because he doesn’t have an argument against the existence of objective morality, only an argument against its motivatingness.
If it was defined by something else, it wouldn’t be our morality.
And “our morality” wouldn’t be morality if it departs from the moral facts.
He doesn’t have a proof that it is, because he doesn’t have an argument against the existence of objective morality, only an argument against its motivatingness.
And “our morality” wouldn’t be morality if it departs from the moral facts.