To use the language of G. E. Moore, it si always an "open question" what morally ought to be done given any statement of what is naturally done or factually the case. To think otherwise is to comit what Moore called "the naturalistic fallacy"--the fallacy of believing that one can derive a theory of what ought to be the case from an account of what is the case. Thus, because of what is (to put it mildly) a certain logical looseness in any account of natural uty, natural law ethical theory often appears arbitrary and confused--an attempt to explain the obscure (what we ought to do) in terms of the even more obscure (moral duties built into nature). When they do attempt to be clear, natural law theorists often offer clarity at the price of uselessness, as when Aquinas offers the following as the first principle of natural law theory: "Do good and avoid evil." One can hardly quarrel with the sentiment expressed here, but one troubled with a moral problem is going to find this piece of highly general advice of very little use. For all these reasons, it is not surprising that natural law ethical theory has often provoked impatience and even contempt from its critics. For Aristotle and Aquinas, the natural law was viewed as a mechanism for imposing duties and giving guidance for the virtuous life. (p. 14)
This is the sort of thing that tempts me to "impatience and even contempt"; for it is this criticism that is arbitrary and confused. First of all, Aristotle is a natural law theorist? Aquinas considers natural law to be "a mechanism for imposing duties and giving guidance for the virtuous life"? While much of what Aristotle says is consistent with, and can be neatly tied in to, a natural law account (hence the ease with which Aquinas does so), natural law theory is not Aristotelian. Aquinas gets the main lines of his account not from Aristotle but from Augustine's considerations on eternal Reason. Further, Aquinas does not consider natural law to be a mechanism for imposing duties. Natural law is not a 'mechanism'; it does not 'impose duties'. It is (if one must speak in terms of duties at all) correct rational perception of duties and what is required for, or consistent with, the virtuous life. Nor can any talk about "open questions" and "naturalistic fallacies" have any affect on the issue, because Moore's question cannot apply; natural law is law, which means it sets out what one ought to do. There is no naturalistic fallacy here, only a recognition that 'human nature' can in fact be an evaluative as well as a descriptive notion and the common principle that moral principles have rational authority.
Second, Aquinas offers the first principle "Seek good and shun evil" not as a counsel for morality, as the author seems to think, but as a basic reference point that needs to be considered in looking at necessity, contingency, and defeasibility in right moral reason. In particular, "seek good and shun evil" is offered not as an earth-shattering novelty, but 1) as a case of moral self-evidence that 2) constrains the basic structure of all precepts of natural law. In other words, the reason Aquinas mentions it is that it defines the field. (See here for the relevant text.) Actual moral guidance comes not with this general principle but with the virtue theory made possible by it. The author's attempt to mock Aquinas's use of the principle in reality supports the Common Doctor's case.
The author then goes on to criticize Aquinas's definition of 'law':
What seems to be happening here is that the concept of ideal or perfect or morally good law is seen as part of the moral order; from this correct insight, a careless slide is made into identifying law itself with a part of morality--the ideality no longer being regarded as a possible and desirable feature of law but as a part of the very meaning of "law." When Aquinas speaks of "being in accord with reason" and "being for the common good," he seems to be making a comment, not merely (and sensibly) about desirable features of law, but rather as part of the analysis of the concept of law or legality--matters of definition rather than evaluation. If this is the view, then it seems immediately open to some serious and rather obvious objections....A dramatic and decisive counterexample to this view, however, is the obvious existence of legal rules that clear thinking would force us to acknowledge as laws even if we believed them to be morally evil. Suppose, for example, that you believe that it is morally wrong for the state to eliminate all considerations of fault in granting legal divorces. Surely you could not reasonably conclude from this that all those persons in a "no-fault" state who claim to be legally divorced are really not divorced at all but are still legally married. (pp. 15-16)
This "dramatic and decisive counterexample" decides nothing. Natural law theories involve what is called "toleration"; i.e., it is often necessary, because of various limits on enforceability, and because human beings fall short of perfect virtue, for the law to tolerate things that are not strictly moral. The most common medieval example of this, interestingly, is prostitution. (The example, of course, is due to Augustine.) The reasoning is that if you outlaw prostitution, then given human immorality "all of Europe would become inflamed with lust." In other words, the medievals felt prostitution needed to be tolerated because things worse for common good (= the good each of us have in common in virtue of being rational and therefore social creatures) would follow if stringent measures were applied against it. So prostitution needed to be tolerated in order to limit the degree to which people could wreak sexual havoc. Further, it was recognized that toleration was a tricky issue; indeed, the whole area of law was recognized to be an area in which perfect certainty or virtue could not be expected, so there would not necessarily be only one way to deal with the difficult problems that arise in legislative matters. It is clearly the case that the issue in allowing divorce is toleration. It is also clearly the case, however, that, contrary to the author, if someone genuinely considers divorce law to be immoral and thus without authority, it is perfectly rational for them to consider people who are divorced to be divorced in name only. And the author says nothing that would actually show that such people are being unreasonable or not engaging in "clear thinking."
As to the "careless slide", Aquinas is quite explicit and deliberate about it. An unjust law is not a law in the sense that a perversion of authority is not the actual authority. An unjust law has several features of a law; it has the appearance of a law; we can call it a law. But an unjust law is missing the essential feature of a law, which is rational authority. If you have something that has no authority then it is not strictly a law, even if it is a law in a looser sense of the term. What gives a law authority? The source of authority in human practical matters: the basic principles of practical reason, without which practical reasoning is not possible, and what follows from them. In other words, natural law. Natural law is a way to connect human law and practical reasoning; and, in fact, it is the only way that has ever been proposed that stands alone. Other attempts can be shown to make implicit appeal to principles that on closer investigation start to look an awful lot like precepts of natural law.
There are many rules in any society that are surely laws but are just as surely morally neutral--e.g., some law requiring that one have one's validated registration tag on the auto license plate prior to March 1. Aquinas sensibly admits that such rules are laws, but the degree to which the admission is compatible witht he literal wording of his definition is unclear. Such rules, though no doubt, consistent with the common good, are not obviously for the common good in the sense that laws prohibiting murder are clearly for the common good. (p. 16)
This is to no purpose; that registration regulations are not for the common good in the same way that laws against murder are not for the common good does not show that they are not for the common good in a different way. And, indeed, if they are not for the common good in some way, what in the world is their point?