Andy has responded to my response to his response to my post on Austin on natural law. Unfortunately, I don't have time for much in the way of a response (it would be the case that all the interesting discussions are happening right when I can't afford the time), but here are my basic points, sort of bulleted and put forward without much development. Apologies if it seems a bit abrupt at points.
1. On the issue of Islamic terrorists, I think it is false to say that their motivation is "Allah wills it"; motivations take the form of ends, and their end is not "Obeying the will of God is good" but "Pan-Islamic civilization should be renewed to its rightful glory," which requires, in their view, that the West, as the primary obstacle to this renewal, be brought down. This is a utilitarian end. But more importantly it is blatantly false to say that religious fundamentalists are the textbook case of natural law theorists; religious fundamentalists all in fact reject natural law theory when it is proposed to them. The rejection of natural law follows strictly from principles of religious fundamentalism, because religious fundamentalism can't allow the account of reason and authority natural law implies. To treat religious fundamentalists as if they were natural law theorists is simply to make the silly mistake of thinking that everyone who talks about the law of God means the same thing by it. The idea that Osama bin Laden is a natural law theorist is utterly laughable, and could not conceivably be supported by a genuinely rational examination of natural law jurisprudence.
2. I don't understand what is meant by saying that moral authority is neither real authority nor practical authority, particularly since you deny that the law obligates, which implies that it has no authority at all, it simply exists.
3. The doctrine of toleration is not that we should obey laws that are unjust but that we should not impose laws merely because they would be just.
4. It is clearly false to say that natural law theory uses the term 'law' obsoletely; the term 'law' is clearly still often used in this sense, and it makes no sense to call a usage obsolete that still is common.
5. Insofar as it gives away the issue of authority to law, legal positivism cannot form an opposition to natural law in the way Bentham and Austin make it, because it has then simply confined itself to facts about law rather than to issues about the authority of law.
6. When I wrote "If we apply the word 'law' to it (as we usually do), we do so equivocally" you wrote:
But we don't do so equivocally. That's an empirical fact of language. We don't put scare quotes around "laws" which we believe aren't binding. People don't assert that abortion is really not legal; they agree that it is legal and assert that it shouldn't be. People don't assert that marijuana use is really legal; they agree that it's illegal and assert that it shouldn't be.
This makes no sense; since it is simply false that we only equivocate when we use scare quotes, equivocation being a logical issue and not an empirical fact about language, I don't know what you mean here. People do assert that abortion is really not legal; you can find Catholics who will say so. They will also allow that there is a sense in which abortion is legal. This is the whole point of saying that it is an equivocation: that people use it in the sense you are recognizing above does not imply that it is the only legitimate use of the word, nor does it mean that legal positivists are not equivocating when they criticize natural law theory (in fact, it can fairly easily be shown that most of them are, because they don't usually take much trouble to connect their use of the term 'law' in their criticism with the use of the term 'law' in natural law theory).
7. Austin's belief that natural law theory is founded on revelation is simply confused; it is a form of intellectualism about practical reasoning, which was assimilated by a particular sort of theology as being, independently, in conformity with that theology's principles.
8. Accordance with right reason is determined by reasoning. Sometimes the relevant reasoning will be utilitarian; in fact, historically the early roots of utilitarianism are arguably a split off from a particular set of considerations in natural law theory or related views. If this means there is no conflict, it is the utilitarian's fault for trying to pick the fight in the first place.
9. It is simply false to say that "An unjust law is not a law" is empirically false, because it is a normative claim that in natural law theory presupposes the recognition that 'law' means several different things. You keep saying that it is empirically false: my challenge is, let us actual see the empirical proof so we can make sure it does not depend on an equivocation or mischaracterization. That we use the word 'law' for unjust laws does not contradict the principle enunciated, for instance; so what is this empirical proof?
10. Since natural law theory is primarily a position about the basic principles of practical reasoning, your argument that King's argument is purely utilitarian fails. This is a problem with most oppositions to natural law theory, which I noted in a post several months ago: they are not rejections of natural law's basic principles but a restriction of them. What you are arguing for is not the rejection of natural law theory but a purely utilitarian natural law. To reject natural law consistently requires the denial that there are any basic principles of practical reason as such; systematic utilitarianism usually does not insofar as it is put forward as a general account of good moral reasoning, because it posits at least one such basic principle. (Nonsystematic or occasional utilitarianism, which does not put forward such a general account, does not depend on such basic principles, but is a simple appeal to consequential ends without attempting to justify this by basic principles of practical reason.)
[It occurs to me that part of the problem is that some things are being treated as principles of natural law theory that actually are conclusions. The principles of natural law theory are simply principles about the natural of practical reason; all the most distinctive aspects of natural law theory are either conclusions drawn from these principles (the notion of moral law, the relations of moral law and positive law, etc.) or are drawn from the doctrine of divine providence (natural law as the law of God), a doctrine that is not entailed by the principles of natural law theory, but which, in certain forms at least, entails that there be natural law. Thus natural law in itself is an account of the principles of practical reason; on supposition of divine providence we also say that these principles of practical reason are the law of God written on the human heart, and thus natural law has a thriving place in theology as part of a (broadly Augustinian, or something analogous to it) theology of providence. Now, unless legal positivists are going to deny either that they reason practically or that their practical reason involves basic practical principles, their fussing about morality and law or about the law of God does not and cannot get at the heart of natural law theory, which are its principles about the nature of practical reason. It is from these that the conclusions are drawn; so natural law theory can only be toppled by presenting a better account of practical reason, which does not involve appeal to fundamental rational principles. Legal positivists do not do this as legal positivists; which is why most of their arguments beg the question against the natural law theorist. Typically, when they do put forward arguments that don't beg the question, it isn't as legal positivists, but as something else; in other words, they really reject natural law's connection of morality and law not on the basis of their account of law, but on the basis of their rival ethical or psychological theory.]