I cannot forbear adding to these reasonings an observation, which may, perhaps, be found of some importance.
Nothing too difficult here. Hume has just finished with a long series of arguments (some of them very interesting) arguing that morality cannot consist in any relations, nor in any matter of fact discoverable by reason alone, comparing virtue and vice to secondary qualities. Just as early modern philosophy of nature rejected the idea that sounds, colors, and heat and cold were qualities in the objects rather than qualities in the mind, so, Hume wants to say, progress requires taking virtue and vice to be qualities in the mind rather than qualities in the objects. This sentence links this argument to this, but does tell us (1) that it is at least a slightly different argument (it is an additional observation); and (2) that it could turn out to be important.
In every system of morality, which I have hitherto met with, I have always remark'd, that the author proceeds for some time in the ordinary way of reasoning, and establishes the being of a God, or makes observations concerning human affairs; when of a sudden I am surpriz'd to find, that instead of the usual copulations of propositions, is, and is not, I meet with no proposition that is not connected with an ought, or an ought not. This change is imperceptible; but is, however, of the last consequence.
This should be seen as Hume's own diagnosis. One of the difficulties people have always had with interpreting Hume in this argument is that nobody actually proceeds explicitly in this way. But that is, of course, part of Hume's point: the change, while extremely significant (of the last consequence) is nonetheless not heralded by anything (imperceptible). Notice the two kinds of is/is-not proposition Hume explicitly singles out: the being of a God and observations concerning human affairs.
For as this ought, or ought not, expresses some new relation or affirmation, 'tis necessary that it shou'd be observ'd and explain'd; and at the same time that a reason should be given, for what seems altogether inconceivable, how this new relation can be a deduction from others, which are entirely different from it.
Lots to consider here. First of all, notice the double recurrence of the word "relation". 'Ought' expresses a different relation from 'is'; and the puzzle is how this new relation can be inferred from other relations. This ties in, of course, to the context of the passage: Hume is arguing against views that hold that morality consists in relations of ideas.
Second, notice what Hume does not say. His argument is that if 'ought' expresses a relation, and if 'is' expresses a relation, then, given that they are entirely different relations, if you try to infer 'ought' from 'is' you would have to explain how you could do so, because it "seems altogether inconceivable" how a relation could be inferred from an entirely different relation. Hume himself never explicitly says that you can't derive an 'ought' from an 'is', and the reason for this is a fairly straightforward tactical reason. We know from the rest of the section, of course, that Hume doesn't think any explanation could be forthcoming. But simply telling readers this would not do what Hume wants to do, which is made clear in the last sentence of the passage. That is, he wants to force his readers to face squarely the problem with moral rationalism:
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 wou'd 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 perceiv'd by reason.
The first and most important thing here is to recognize what conclusion Hume wants us to draw from the problem he has raised; and Hume makes it quite clear: it lets us see "that the distinction of vice and virtue is not founded merely on the relations of objects, nor is perceiv'd by reason". Hume has throughout the section argued against precisely these two possibilities, i.e., that morality consists entirely in relations of ideas or is the kind of matter of fact discoverable by reason alone. (And what are the matters of fact discoverable by reason alone? Matters of fact reached entirely by inference; which, given what Hume has argued previously, can only be by causal inference, the only form of inference available to reason that can conclude to a matter of fact.) Hume is still on topic.
One of the things that has often puzzled people is the sudden mention of "all the vulgar systems of morality". He actually mentioned them before (without the 'vulgar'). It's important to note that Hume does not mean anything derogatory by the word 'vulgar' itself; he regularly uses the term as a synonym for 'popular', where that is contrasted with 'philosophical'. He doesn't specify, but from other things Hume says elsewhere I think we can pin down fairly precisely what he has in mind.
The Whole Duty of Man ('laid down in a plain and familiar way for the use of all, but especially the meanest reader,' as the subtitle goes) was an anonymous Calvinist work, perhaps by Richard Allestree; Hume was intimately familiar with it, having read it avidly when young. His thought also seems to have gone back to it more than once as an example of the sort of foolish thing he had had to escape: he mentions it to Boswell almost at the end of his life, saying that he had made a catalogue of vices from the list at the end of the book and regularly examined his life against it. He also mentions it to Hutcheson, saying that he preferred to take his virtues from Cicero's Offices rather than The Whole Duty of Man (something of which Hutcheson might well have approved if he had thought that Hume was interpreting Cicero correctly -- which he didn't). The Whole Duty of Man argues that some duties are "stamped upon our Souls" and known by "Natural Light," that is, by reason. And throughout the work we see exactly the slide Hume notes: a duty is laid out and the reason for it given, and the reason is always something like the fact that we are creatures of God (an 'is') from which the 'ought' is supposed to be inferred. Thus rationalism is not confined to the learned and scholarly: it can be found even in a text whose express purpose is to present an account of duty for "the meanest reader".
So we've gone through the argument and noted some of the more noteworthy points. In the next part in this series I will draw some conclusions.
