In ethics, there are two ways to take human nature seriously. The first is to regard nature as the design of a supernatural intelligence; you take it seriously because you take God seriously. The other is to regard nature (in a physical or material sense) as the reason for all there is. Here you ascribe to matter -- or to some property, process, or aspect of matter -- the ontological status that theists ascribe to God Himself. Natural lawyers follow the first way; naturalists follow the second. Similar names -- radically different meanings.
Nature means something different to the naturalist than it does to the natural lawyer. It has to. He cannot view it as a design, because in his view there isn’t anyone whose design it might be. What is, just is. This is rather unsatisfactory, for no one seriously maintains that the universe had to be just the way it is. There might have been fewer stars, or more. There might have been creatures like us, or there might not. There might not have been a universe at all. Nature, then, is a contingent being, not a necessary being like God, and contingent beings need causes. The naturalist rejects this line of reasoning, or at least limits it. He might concede that each thing in nature needs a cause, but he denies that the entire ensemble of things needs a cause. This exception seems suspiciously arbitrary.
It is easy to see how the first approach can ground ethics. If God Himself is the Good -- the uncreated source of all being, all meaning, and all value in created things -- then inasmuch as his goodness is reflected in the inbuilt purposes of our own design, these purposes are normative. Consider, for example, the inclination to associate in families. This is not the same as a mere desire to do so; indeed, we have conflicting desires, and some people would rather be alone. It would be more accurate to say that we are made for family life, that fitness for family life is one of our design criteria. For humans, then, the familial inclination is a natural inclination. When we follow this inclination we are not acting in the teeth of our design, but in accord with our design. Family is not a merely apparent good for us but a real one, and the rules and habits necessary to its flourishing belong to the natural law.
Or consider the universal testimony of conscience against murder. This is more than a matter of guilty feelings. No one always feels remorse for doing wrong, and some people never do. Nevertheless, the wrong of deliberately taking innocent human life is acknowledged at all times and everywhere, and this too belongs to the natural law.
Notice that both examples concern design. The former concerns the design of the inclinations, as apprehended by the intellect. The latter concerns the design of the intellect itself – we could say of its inclinations -- for we are so made that there are certain moral truths we can’t not know.
How the naturalist view could ground ethics is hard to see. If material nature is all there is, then how could actions have nonmaterial properties like right and wrong? How could there be true moral “law” without a lawgiver? Perhaps it would be like the “law” of gravity -- a pattern that we cannot help but enact, a force to which we cannot help but yield. But in that case, “you ought to” would mean the same thing as “you do.” Stones do not deliberate about whether they “ought” to fall.