I think it's interesting that the snippet about Hume gives us three forces for the explanation of our morals as well as claiming to explain the reason we believe our morals to be objective (insinuating that Hume does not believe this). If there are only three universal forces for explaining human ethical belief how does Hume explain the diversity of ethical beliefs between cultures, or even between individuals? Also if these three forces account for all ethical beliefs can't we also say that they function in a sort of pragmatic objectivity. Sure, these ethical beliefs might not exist "out there", and may therefore not be definitionally "objective", but universally existing "in us" is probably the closest the we can get to objectivity.
I also think that if Hume believes these to be the only forces which give rise to our ethics he might be participating in some pretty hefty reductionism. But, again, these are only snippets and I'll have to wait for the reading to comment further.