But the matter does get critical scrutiny from time to time, such as in a post I've just come across from Catholic philosopher Alexander Pruss of Baylor University. Pruss uses his blog to germinate and sketch philosophical ideas, with which he positively teems. Here, the general thesis is that "We have prima facie reason to conclude from the fact that something ought to be so that it is so." If that thesis can be further developed, disseminated, and defended, it would go a long way to removing what I consider a major obstacle to doing sound moral philosophy.
It seems to me that Pruss' argument is valid, or can be made so on suitable restatement. What interests me is what his thesis, if true, says about philosophical argumentation generally. To say that one has "prima facie reason" to infer such-and-such is, I take it, to say that one is within one's epistemic rights to infer such-and-such unless a clear defeater is known. I'm uneasy about non-demonstrative arguments of that sort, for their conclusions are of interest less for learning what is the case than in learning what we have justification for believing is the case. It is possible to be justified in believing something that is not actually so, if one lacks grounds for inferring that it isn't so. But the interest of that lies more in its utility for defending one's intellectual virtue than in its utility for discovering facts.
And yet, Pruss' ambitions for this thesis are apparently bigger than that. Thus he muses: "It is an interesting question whether the is-from-ought inference is at all plausible apart from a view like theism or Plato's Platonism on which the world is ultimately explanatorily governed by values. There may be an argument for theism (or Plato's Platonism!) here." If that musing turns out to be true, then the truth of the original thesis would itself serve as a premise in an argument for the existence of God, as well as removing a major de facto obstacle to natural-law and virtue theory in ethics.
I need to think about this more. Do any readers have thoughts in the meantime?