How does 'Is–ought' 'subvert all the vulgar systems of morality' ?

Question

Source:: Hume discusses the problem in book III, part I, section I of his book, A Treatise of Human Nature (1739):

... But as authors do not commonly use this precaution, I shall presume to recommend it to the readers; and am persuaded, that this small attention would subvert all the vulgar systems of morality, and let us see, that the distinction of vice and virtue is not founded merely on the relations of objects, nor is perceived by reason.

Am I right that this small attention means the problem of leaping from [is and is not] to [ought and ought not]? Even if so, how does this subvert all the vulgar systems of morality?

Answer 1

Hume is using vulgar in its original sense, meaning "common." His claim is that all common systems of morality (he's probably largely thinking of Christian morality here) muddy the distinction between statements of fact like "water is wet" and statements of value like "water is good." The overall thrust of his argument is against the rationalist concept that the right moral judgment can be produced by reason (as usual, his main antagonist is Descartes). In building this argument, he leans on his empirical commitment to the idea that all statements of fact must be founded on sensory evidence.

Answer 2

'Vulgar' here shouldn't be taken in its usual pejorative sense; but in the pejorative sense of philosophy - of not being soundly (ie foundationally) based.

Hume's is-ought distinction, or so he claims, amplifies this; but this only if one believes principles should be 'one'; but not, perhaps if one thinks in contraries - the inner world opposed to the outer; microcosm vs macrocosm; or by a principle, that is one but has contrary manifestations - the noumenal world or Schophernian Will.