Tod Kelly sees an inherent danger in the theory of natural law:
For if you believe that there is an absolute Good and Evil — and that these absolutes are knowable — then you tend to favor the systems that most clearly and staunchly define them. You are pushed, ultimately and over time, toward a place where doubt does not exist and is not welcome. And a lack of doubt concerning what God considers Good and Evil public policy always leads to the disenfranchisement of the Other — if not in individuals, then certainly in groups. For those who consider themselves warriors of The Truth, doubt is synonymous with moral relativism even though they are in fact disparate things. One can believe that there is good and evil and still doubt exactly where to draw the line. And make no mistake: embracing that doubt is necessary in a pluralistic society.
What Tod describes here clearly happens, but I’m not convinced the culprit is the belief in natural law. To believe that there are knowable absolutes of good and evil does not imply absolute certainty that you have the last word on what they are. Saying such absolutes are knowable means simply that they can be apprehended by reason and can therefore serve as a guide to reason. If the precepts of natural law were unknowable, then you’d have nothing on which to base a natural law theory and develop natural law arguments. You wouldn’t know where to begin and where to go. An ethical theory has nothing to say if its principles are unknowable. This goes for any kind of ethics: utilitarianism, consequentialism, Kantianism, you name it.
The first precept of the natural law, classically understood, is that good is to be done and pursued and evil is to be avoided. The good is that which all things seek after—the end to which they are naturally inclined. Naturally here does not mean whatever occurs in nature, as if what happens in nature is by definition morally good. When classical natural law theorists like Aquinas speak of the nature of something, they are speaking of the kind of being that it is. Ontologically, not biologically. So, for example, you and I are rational animals—living beings capable of knowledge, reason, and free choice. We are naturally inclined to preserve our lives and our livelihoods. We seek knowledge of ourselves, one another, and our world. We desire freedom. According to the natural law, these things—life, knowledge, rationality, freedom—are among our natural ends and are therefore included in the good. A secondary precept of natural law, then, would be that we ought freely to know the truth and to live according to it.
All precepts of the natural law follow the logic of the first precept, so developed natural law theories tend to be complex teleological systems. However, not everything the natural law says is absolute. In its first principles, the natural law cannot change without the whole structure collapsing, but you can add or subtract to its secondary principles, and should, as our knowledge about human nature changes, deepens, and expands. Aquinas said as much, though no doubt he’d disagree with some revisions I’d suggest. In my favor, the natural law leaves a lot of room for diversity of judgment and doubt about natural ends. Admittedly, its close ties to the perspectives of scholasticism and later the Enlightenment have kept it grounded to these foundations, and too often appeals to natural law are attempts to pass off beliefs and prejudices in regal philosophical clothing, but its heart is open to pluralism.
In this vein, I like what Aragorn says to Éomer in The Lord of the Rings when the bewildered horseman asks how anyone is to judge what to do in such a time of marvels and strange occurrences. “As ever he has judged,” Aragorn answers, explaining that good and ill don’t change with time or place, but that they must be discerned whenever or wherever people find themselves. Aragorn appeals to a natural law, but he doesn’t tell Éomer what judgments he ought to make. He counsels discernment precisely because the natural law isn’t a laundry list of moral norms covering every possible situation and circumstance. It’s a guide to the discernment that each person has to make in her unique time and place.
For my part, I tend not to appeal to natural law as it has historically relied on philosophical assumptions I don’t hold. To my way of thinking, the meaning of human nature owes as much (maybe more) to the productivity of language and thought as it does to what we apprehend in the study of actual human beings. Natural law, so to speak, begins as an interpretation of nature. It’s rooted in human thought about nature, not in nature itself. It’s a construct and therefore open to deconstruction.
Regrettably, natural law has been used by people in positions of power to oppress individuals and groups; but the problem isn’t natural law, either in its classical conception or my postmodern spin on it. Natural law thinking could just as easily serve the goal of equality. The problem is the uncritical willingness of those with power to remake the world in their image, others be damned.