Hume was not, on his own terms, so dogmatic about the problem. He ends the oft-quoted passage about the is/ought gap by saying:
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.
By themselves, these statements do not imply that deriving an "ought" from an "is" is absolutely impossible (Hume, as adventurously skeptical a thinker as he was, I would not expect to have come down so strictly on every conceivable such derivation; and there is a sense in which applying his own bundle-theory of objects generally, and particularly of subjective (human) objects, does mean adverting to relations between objects to characterize moral attitudes—but now we are speaking of either moral relations or relations-in-general trivially). We have, as a philosophical community, tended to extract a syntactic point from the appearance of the gap; then the naturalistic fallacy might be interpreted as the syntactic gap's semantic accompaniment.
Now, as for teleology, one question would be whether Thomistic ethics, say, is "vulgar" or not. Offhand, I assume not: Aquinas was a very elaborate and detailed theologian, who paired teleological with other normative judgments in representing his overarching system. If Aquinas had spoken with Hume, and if Hume had cautioned Aquinas about the is/ought gap, I expect that Aquinas could have easily responded with something like, "Then I will not use the word 'ought' [or rather, the Latin cognate] when it would cause my theory trouble to use it; the mere word 'ought' is not so important." (Actually, Aquinas would've put "on the contrary" in there somewhere, but nevermind that, I haven't read Aquinas' work enough to know his style the way I know Immanuel Kant's or John Rawls', say.)
Alternatively, taking Aquinas' system as bracketed on the lower end by the analytically justified imperative, "Do good and avoid evil," and on the upper end by, "The ultimate power of goodness in all possible reality has made our apprehension of Its presence into our ultimate goal," we can see that Aquinas both grounds, and diadems, his system with "oughts" that stand over the teleological semantics in play. For if God were not Good, say, then it would not do to claim that the beatific vision is justifiably promised unto us: the visio malefico de summum malum would hardly be what we "ought to aim for" even if Evil had created our souls and our world (if God were somehow Evil, then).
The upshot is that final causality is not really a synthetic bridge-principle over the gap: I can create a sword to kill, but it is not then that "the sword ought to kill" or even "I ought to use the sword to kill." If this were so, then it would not be that "is" and "ought" had been connected, but they would be conflated, which would suggest either equivocation or at least suppression-of-premises when it came to meaningful argument. At best, we might compare appeals-to-final-causality to prima facie duties/pro tanto reasons: something's-having-a-purpose might be a partial right/good-making property, but not enough to fully determine the rightness or goodness of fulfilling such a purpose.
If we want to apply this to questions of human sexuality, say, we might think:
- God predestined evolution to program sexual desires into humans so that humans would have a motive (pleasure) to engage in the bizarre ritual of conception.
- Some people were also predestined to be sexually attracted to others with whom they could not perform said ritual.
- Such people then have two purposes behind their sexual capacity: a general purpose based on reproduction and a particular purpose based on expression of romantic affection (for example).
- If they were to aim to fulfill only the one purpose, not only would they probably not fulfill it, but they would frustrate the other purpose.
- So why not permit them to fulfill the purpose that they can fulfill without feeling disgust and loneliness in return? If they are not, by doing so, frustrating other people's acceptable purposes, then doesn't the weight of teleological reasons favor said permission?
But I should observe, then, that this would be another possible problem with invoking final causality to bridge "is" and "ought": very well, you have gained the moral world, and now you can send your moral soul anywhere in that world, i.e. teleology is amorphous enough to be manipulated, as a premise, to trivially justify-or-condemn most anything under the sun. For example, if the sin in Eden recreated human nature with evil now internal to that nature, do we not have grounds to think that hatred of God is somehow part of our natural purpose, even if love is as well? Then the next problem is not triviality, but nullity: because we can be set contradictory purposes, even by God (let us suppose), then if some teleological "is" were enough to base a deontological "ought" upon, would we not end up losing the moral world, and our moral souls, to the void of inconsistent final causes?✞
✞We should not forget, either, that if a final cause really is a cause, then it really has an effect, and it doesn't have this effect contingent upon our will, but should override our will to guarantee that we act so as to fulfill the given purpose. Otherwise, a final cause is not a cause as a determinant of action but is yet an ideal for action, and then whether it is a justified ideal is no different a question than, "Is this ought-statement true?" in the first place.