Another reason natural law theory is not popular, though, is almost certainly its metaphysical associations. So I've been reading with some interest William Brewbaker's "Thomas Aquinas and the Metaphysics of Law" (at SSRN). I'm very sympathetic to the argument, and think the overall argument (that whether or not you accept the metaphysics, it raises important questions you can't really afford to ignore) is more or less right. But I think the argument goes off the rails here and there in the details.
One of the claims Brewbaker makes that I think is clearly wrong is that Aquinas thinks that law is a natural kind. Brewbaker rightly notes that Aquinas does talk about law's essence; but it is clear, I think, that the 'essence' here is just what you can define. The mere use of the term does not imply anything about whether the essence belongs to a natural or an artificial kind, or whether it forms a natural genus or not. And there is, in fact, good reason for thinking that law is not a natural kind for Aquinas. To be a genuine natural kind, 'law' would have to be univocal across the various species of law (eternal, natural, positive). However, Aquinas is very clear that the species of law are related to each other by participation -- positive law is a participation, or 'parceling out', of natural law, natural law is a participation of eternal law. Now participation can allow for univocity only when the participating is just an instance of the participated. It's clear, however, that Aquinas doesn't think positive law is just an instance of natural law. Positive law adds determinations that are not in natural law, because it factors in questions of utility and enforceability. So positive law is related to natural law by participation, but not as a species participates its genus. This is only confirmed by the fact that Aquinas locates the unity of law in order to a common good -- again, the relation is analogical rather than univocal. Brewbaker reads this as an equivocation on Aquinas's part, but it seems to me to be more plausibly a case in which one of Brewbaker's interpretive assumptions is wrong. Thus law does not appear to be a natural kind. This is not to say there isn't anything natural about it (there are lots of things in Aquinas's metaphysics that are natural but not natural kinds -- transcendentia, for instance), but it is one thing to be natural and another thing to be a natural kind.
And attributing to Aquinas the claim that law is a natural kind leads Brewbaker to say some odd things. For instance, he asks the following questions on p. 28:
Humans, horses, trees, rocks, and their essential characteristics exist in their own right apart from human will. The same is true of eternal law and natural law. Positive law (human law, in Thomas’ terminology), on the other hand, is a human creation. Can a cultural artifact like human law be part of a natural kind? Or might it be part of law’s essence to exist apart from human will? Put another way, is human production of law consistent with human law’s status as law?
But this is only a puzzle if we assume that human law is part of a natural kind rather than (as Aquinas says it is) something devised by reason for purposes of justice and utility.
Another odd thing Brewbaker says in this connection is that "Thomas has no trouble with the idea of natural kinds because he believes God purposefully created the world." But while it's true that Thomas has no trouble with the sort of things that we call 'natural kinds', the reason for it isn't that 'he believes God purposefully created the world'. Suppose that Brewbaker is right, and Aquinas thinks of law as a natural kind. It still doesn't follow that 'God purposefully created the world' renders 'law is a natural kind' unproblematic, because law's being a natural kind would not be explained by God's purposefully creating the world. One of the kinds of law is eternal law itself, the divine reason insofar as it is ordered to practice. And this cannot be explained by creation. (The fact that one of the kinds of law is divine reason, however, is yet another reason to doubt that law forms a natural kind rather than being something that can be attributed analogously to different kinds of things.)
Now, it must be said that Brewbaker recognizes that 'law' is applied analogically rather than univocally; but he doesn't seem to see that this problematizes his claim that Aquinas attributes natural-kindship to law. Surely a condition for being a natural kind is being categorical; things that aren't categorical -- like 'good' or 'true' -- aren't natural kinds. But the sort of analogy that we find in the case of law shows very clearly that 'law' isn't categorical for Aquinas. So he isn't committed to the view that law is a natural kind.