Brock Sides writes in reaction to my last post:
As a former philosophy PhD student, I'd like to point out that the "is-ought gap" is not a distinctly "post-modern" thesis. It originates with David Hume, in the Treatise on Human Nature, Book III, part 1,section 1:
"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 ways 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. 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."
And the term "naturalistic fallacy" was coined by G.E. Moore, who wasnot post-modern by any stretch of the term.
As to the opposing thesis, that the is/ought gap can be at least partially bridged: whereas Rawls would be the most famous 20th century exponent of that view, I think Kant deserves some credit for originating the project.
So if anything, we should see this disagreement not as one between the Enlightenment and the Postmoderns, but as between two giants of the Enlightentment, Hume and Kant.
So I then asked him whether Enlightenment thinkers who posited the is/ought gap and the naturalistic fallacy lead the way to Nietzsche and his throwing out the possibility of any type of objective morality (whether from Reason—Nature on the one hand or Biblical Revelation on the other) altogether. And he replied:
I'm not qualified to say anything about Nietzche's influences (I specialized in early modern and 20th century analytic)….
I can add that the post-modern innovation was not so much taking note of the is-ought gap and denying objective morality, but in taking that Enlightenment thesis about ethics and turning it onto epistemology and science.
Also, I'll note that moral subjectivism (or even moral skepticism)does not follow from the is-ought gap. Bridging the is-ought gap would defeat ethical subjectivism/skepticism, but the is-ought gap is logically compatible with ethical objectivism and ethical knowledge. (See G.E. Moore for the most notable, and clear, proponent of both theses.)