I've been thinking more fully about the issues involved in legal positivism as opposed to natural law theory, and think I can make a clearer presentation of my point. The issue I have with legal positivism is that there seems to be no reason that would justify a natural law theorist in taking up legal positivism. Consider some of the advantages, prima facie at least, natural law theory has over legal positivism:
There appear to be no facts taken into account by legal positivism that cannot be taken into account by natural law theory in its discussion of positive law; natural law theory does not deny these facts but puts them in the whole context of practical reason.
Natural law theory is a more comprehensive theory of law, since it considers law not only in terms of legal process and reasoning on the judicial side, but also on the side of legislators, on the side of those trusted to enforce the law, and on the side of citizens subject to the law. (It has tended historically to be most interested in this latter, but this is not necessitated by the principles of natural law.)
Natural law theory clarifies the relation between morality and law; legal positivism (considered simply in itself) does not.
Natural law theory clarifies the relation between law and principles of practical reason; legal positivism (considered simply in itself) does not.
Natural law theory provides principles according to which a rational justification for the existence of positive law, its processes, and its mechanisms, as well as for the extension of these processes and mechanisms into new areas, is possible.
And so forth. So what superiority does legal positivism have over natural law theory? Andy has taken the position that the first of the advantages above is not the case, that, in fact, legal positivism is more in line with the empirical facts. But it is difficult to see what facts about positive law there would be that natural law theory would not be able to take into account in its account of positive law. It cannot be facts of language, since natural law theory admits those; its claim is not that this is the way people speak, but that this is the way their thought about law can be rationally inconsistent. And whether or not this is so is not an empirical fact in any obvious sense. Further, if it were about the way people spoke, this would not clearly favor legal positivism, because people sometimes speak in ways that unambiguously seem to presuppose natural law theory, too. And it certainly would not have justified Austin; after all, the person he was criticizing was no less than Blackstone, who takes it for granted that people did, in fact, speak this way, and given Blackstone's popularity there is no reason to think he was alone in this. Further, legal positivists have not, as far as I am aware, ever really made an effort to fix claims about how the word 'law' is used as an empirical fact. They might appeal to it, but they do not establish it. In matters of language legal positivists have from the beginning been reformers, not researchers.
Nor can it be facts about the legal process. Consider, for instance, Austin's Court argument against Blackstone, in which he characterizes Blackstone's position as stark nonsense because courts impose unjust laws as law. Not only does this beg the question, Austin does nothing to show that natural law considerations did not as a matter of fact play some role sometimes in court cases. I will have to leave it to specialists in early modern law as to whether they ever did; but Austin certainly doesn't do anything to show they didn't. Moreover, not only does it beg the question and not only is it not supported by actual discussion of the facts, it misses the point. Even if courts did not ever consider natural law issues, courts are not the whole of the system of law. Now it seems clear that, whatever position one takes on the courts, Blackstone seems to think it a straightforward fact that the English system of law, taken as a whole, involves natural law considerations, and, again, given Blackstone's popularity we have at least some reason to think there were many people who agreed with him. Austin never proves the fact to be otherwise. Nor does it seem to be the case that legal positivists have ever done the actual empirical work that Austin doesn't do.
Nor can it be that the facts of the legal system preclude the natural law conclusion that an unjust law is in an entirely reasonable sense not a law. For again, legal positivists have never done the work to show that it is so. Indeed, we have at least prima facie reason to think otherwise. As the Canadian Constitution is at least sometimes interpreted, for instance, courts may in certain sorts of cases consider a law contrary to the Constitution simply on the basis of moral principles; however, any law contrary to the Constitution explicitly is precluded from having any force or effect of law, and judges are required to treat such laws in this way. But this means that there is an entirely reasonable sense in which, in these cases, an unjust law would not be a law, and if this interpretation is true, this is a fact about Canadian positive law itself. Even if the interpretation is false, the fact that a legal system could exist on which it would be true should be cause for worry to the legal positivist who reject "An unjust law is not a law" as a matter of legal fact; for law is what people make it, and there is no obvious reason why people could not make it conform to this law. Indeed, there is at least prima facie reason to think that it is already at least sometimes done in some places.
So the legal positivist must allow that as a matter of positive-legal fact, things can go down as the natural law theorist says they should. In other words, legal positivism cannot build itself on an absolute rejection of the possibility that natural law considerations can have a real and even fundamental place somewhere in the legal system as a matter of positive law itself. What, then, does the legal positivist have to hold in order to stay a legal positivist? This: he has to deny that this is anything more than a matter of positive law. In other words, he has to deny at least one of these two principles:
1) There is a moral law independent of positive law.
2) This moral law is a higher law, or law in a more fundamental sense.
But it is clear that the legal positivist cannot do this qua legal positivist, because legal positivism sticks within the positive law itself. So what is really opposing natural law theory is not legal positivism at all; it is something added to legal positivism that denies one of these two principles -- an ethical, metaphysical, or religious theory that goes beyond the positive law in order to deny the context of positive law that natural law theorists claim exists. Such a theory has to oppose positions like natural law theory and Kantianism that allow practical reason a legislative role prior to and morally higher than any positive law. But a position that depends on this sort of theory in order to refute natural law theory is not a theory whose superiority to natural law theory is an empirical matter; it is based entirely on (1) whether the arguments natural law theorists actually put forward stand and (2) whether the sort of theory on which legal positivism really depends in its rejection of natural law theory can really hold up. Whether legal positivism rather than natural law theory is true depends on whether prior ethical commitments can be maintained. Since it is difficult to find any legal positivist discussion of the actual primary arguments of natural law theory (which I have briefly summarized), and given that legal positivists have not proven their prior ethical commitments (and certainly cannot do so simply as logical positivists), I can only conclude that
(1) It does not seem that there is any good reason for a natural law theorist to switch to legal positivism.
(2) It does not seem that most legal positivists, and perhaps that any legal positivists, are rationally justified in their rejection of natural law theory.
[UPDATE: I didn't read "Under the Sun" first, so I missed that Andy had already responded to my much rougher and undeveloped previous post. What he says there requires, I think, some qualification of the claims about Blackstone above, on which my general position does not depend. My primary beef on the whole issue of Blackstone is merely that legal positivists have often seemed to interpret all natural law theory generally through Austin's criticisms of Blackstone; whereas I don't think Austin even does a good job of criticizing Blackstone's rather basic and unsophisticated summary, much less saying anything that would be troublesome for more sophisticated natural law theorists. Even if Austin had Blackstone pegged, though, it would, as Andy says, be a sign of Blackstone's lack of sophistication rather than of any problems with natural law theory itself. Also, Andy's post clears up a misunderstanding about the reason he was appealing to empirical fact, so that should be kept in mind, too.]