Again with the bastardization of 'post-modernism'

First, the standard caveat: 'post-modernism' is such a vague and expansive word as to be referentially useless (although it has other uses - in the language of my hero C.L. Stevenson, it's a very useful persuasive definition).

Often enough, though, post-modernism is used as short-hand method to bundle together several theories that don't necessarily go together: ethical relativism, epistemological skepticism, political tolerance, ethical non-cognitivism, etc. I'm no lunatic about semantic bundling - I do it often myself. For example, when I refer to Christian fundamentalists, I'm bundling several things together, such as Biblical inerrancy, political conservatism, a belief in the legitimacy of Biblically-based law, etc. What's important, though, is that the bundler understand the contingent relationship between the traits being bundled. In other words, just because I bundle those traits together doesn't mean that the three I listed logically entail one another. There could well be a Christian that is inerrantist, is politically conservative, but doesn't think it'd be a legitimate use of political power to enact Biblically-based law.

With that said, here's a good example of 'post-modern' bundling:

But interestingly, Posner and Volokh are non-religious. So they don’t believe in the ultimate Truth of religious teachings either. So ultimately we are left with no objective grounds for moral judgments. That, it seems to me, is postmodernism-nihilism.

This is a fine example of bundling ethical non-cognitivism with ethical relativism. The claim is that P&V aren't able to 'objectively ground' moral judgments (whatever that means), and so are necessarily 'postmodern nihilists' (which presumaby means ethical relativists, dressed in the style of the day per Christian apologetics).

What needs to be explained is how we get from non-cognitivism to relativism, from the claim that moral propositions aren't T to the additional claim that you should be able to do whatever you want.

Typically, when this issue is raised, the objectivist responds: "but if there's no objective grounding for your claim that people shouldn't do X, how can you persuade others that they shouldn't do X?" This is a concern based on the efficacy of moral reasoning. One counter to this is pragmatic: it really doesn't matter whether or not I hew to the belief that morals are 'objectively grounded' (whatever that means); in the real world, people already share many, many moral beliefs. Discourse proceeds from our shared moral commitments, not from a laboriously worked-out system that is deduced from agreed-upon axioms.

Once that becomes obvious to our objectivist interlocutor, the next thing one typically hears is: "well, what do you do when the other doesn't share any of your moral beliefs?" At that point, of course, moral reasoning simply breaks down. And it's important to note that this is a feature of reasoning as such, and not of the non-cognitivist's moral reasoning. All of the Biblically-based ethical beliefs in the world are useless when the other doesn't accept those Biblical axioms. So if you try to persuade the other of the truth of the Bible through historical evidence, and the other simply doesn't believe in historical evidence, the process of reasoning simply breaks down. You can go no further.

So the efficacy of reasoning is necessarily limited to shared presuppositions. That's just a characteristic of all reasoning, and as such isn't a counterargument to ethical non-cognivitism. Or, if it is, it's also a counterargument to every belief system. In other words, we don't point to the existence of people that don't believe in God to disprove the existence of God. That's just an insane argument, as is the objectivist's argument from efficacy. Both oversell the power of reasoning; both conflate the formal limits of reasoning with the truth of what is reasoned.

Ninety-nine out of a hundred times, the argument from efficacy is what is used. I've only had ninety-eight discussions like this, so I have yet to see other arguments deployed.

Update: John from Fake Barn Country (on an aside, the funniest name in the blogosphere) ends on a similar note here:

I don't need to be able to convince a skeptic about the external world that I have a good epistemic reason to believe that external objects exist in order to, say, reasonably believe or know that I have hands. Likewise, I don't need to be able to convince the moral skeptics among us that unredeemed suffering is a good moral (or political) reason to oppose a practice in order for me to have a good reason to oppose preventive war. Anyone who seriously denies that premise (as evinced by both word and deed) needs more help than philosophy or enlightened public discourse can ever hope to offer. [emphasis mine]

Once the limits of moral reasoning are reached, it's the other's problem. If someone seriously questions whether people ought to be allowed to live (other things being equal), then the problem isn't mine anymore; it's theirs.