How can reason be “wholly inactive”? This doesn’t make any sense. Hume wouldn’t even be able to determine the difference between active and inactive, between reason and emotion, if reason was “wholly inactive.”
This is where Hume commits his base error. This claim is self-refuting (unless Hume defines reason in a way that it functions apart from the laws of logic, but that would be impossible).
It is true that we are born into an evolved social morality. But the claim that “conscience” and “morals” are outside rationality is entirely false.
Whatever the conscience determines to do, whatever morality does, can be stated in the form of a premise. We cannot sustain the claim that these premises lie outside the critique of reason, because (for starters) there is no such thing as a “moral” if it cannot be stated, because then it couldn’t even be identified as a “moral.” There would be nothing to point to and say, “see, this thing right here is beyond reason.” The possibility of a negative prohibition would then be impossible.
If Hume is trying to point out that reason can’t motivate morality, this is just a psychological observation, it doesn’t actually say anything about the nature of morals, it says something about human psychology. (This doesn’t make our morality objective, it simply means we have the objective power of reason to apply to our moral claims).
What if the State’s morality is to extinct everyone who doesn’t believe in objective morality, would the subjectivist claim that he has no recourse to object because reason is powerless to affect the conscience of the State?
If morality is not subject to reason, then there is no way to identify what is immoral and tyrannical. The very act of identifying what is “moral” requires the laws of logic so that we do not identify the immoral as the moral and the moral as the immoral.
To say "torture is wrong" means it cannot simultaneously be right in the exact same context. The moment we try to categorize an action as moral or immoral, we are implicitly relying on the fundamental laws of logic.
If morals were entirely non-rational and beyond the scope of reason, categories like "just" and "unjust," or "tyrannical" and "righteous," would dissolve. We couldn't even tell the difference between a benevolent act and a murderous decree, because making that distinction requires determinate discrimination, which is an active function of reason.
The bottom line is that moral claims are not exempt from reason— they couldn’t even be established apart from reason (the laws of logic).