Question

Good evening!
Hume argues that one cannot derive a norm from a fact. And I don’t understand: after all, he doesn’t deny that one can determine in principle what ethics is—otherwise there would be no subject at all. Rather, he argues that although we may perhaps be able to define conceptually what the proper fact is, we still cannot derive from that that we ought to act on it. My question is: how is that different from a fact such as that a person must eat, where the very fact obligates him, and he can only choose to starve and suffer? And how is that different from an ethical fact, which, granted, he can choose not to carry out, but it will cause him frustration?
In other words: insofar as it is possible to determine what ethics is, what is the fundamental difference between it and any physical fact? Just as here a person can choose not to act accordingly, so too there. [Let me emphasize: my assumption is that Hume agrees that in principle one can know what ethics is, and is only asking how it can obligate; otherwise, what did he add? It is obvious that if there is no ethics, there is nothing obligating. Even more so, the claim that it is impossible to find what ethics is is the postmodern claim, not something Hume introduced (despite the relativism…).]
Thank you very much!