Natural law has been the primary scaffolding behind Catholicism’s critique of much in postmodern America. From its condemnation of abortion and homosexuality to its arguments in favor of the existence of God, the Church maintain that there are ethical principles universal to rational man. These principles are not part of a supernatural revelation, but are rather available to anyone with sufficiently developed rational faculties; and although culture might condition how these principles are expressed their content is not culturally dependent. It seems natural to me that if these claims are correct the content of natural law must be a proper object of empirical research, rather than simply a subject of rational speculation. However, despite what in my opinion is an unavoidable conclusion, I have yet to see any Catholics putting forward a truly empirical treatment of Natural Law. It is unclear what this means, but at least one possibility is that the results of such an empirical investigation would be a very messy ordeal for proponents of Natural Law, and that it is easier to propose the universality of certain ethical principles than to demonstrate it. Furthermore, I hypothesize that any attempt to formulate the essential or primary content of Natural Law would either be contradicted by empirical observation or would be so empty and vague as to count as tautological.
First, it is necessary to admit that Natural Law proponents readily accept that different cultures have different ethical norms. For one thing, there is a degree of leeway within the principles of Natural Law; as Chesterton put it (and here I am paraphrasing from memory), “different cultures may disagree about whether one can have one wife or many, but all agree that one cannot simply have sex with whoever they want.” Putting aside the lack of empirical evidence for this statement, I can readily agree with the sentiment that a principle of Natural Law might be less a statement of extremely specific rules, but rather general categories or “first principles” of ethical behavior that might be embedded in different culturally specific rules. Furthermore, these rules might vary in their degree of adherence with the first principles: one culture might take very stringent or strict stances on such a general code as “do not harm your neighbor,” while others might very well be more relaxed in their definition of what counts as “harm.”
But Natural Law proponents also have another rather important distinction between what might be called primary or essential principles of natural law and secondary principles. For example, if we hypothetically counted “do not harm your neighbor” as a primary principle of natural law, we might count “do not steal your neighbor’s property” as a secondary principles: that is, it is a principle stemming from the concept of “do not harm your neighbor,” and it is at least conceivable that a culture might follow principles of “do not harm your neighbor” while not ascribing to “do not steal your neighbor’s property:” in their case, for whatever cultural or historical reasons, they might never have counted the theft of property as particularly harmful (some may even have considered the ability to steal their neighbor’s property as a sign of virtue and strength, such as the Germans that Thomas Aquinas describes in some of his treatment of Natural Law). Natural Law proponents allow for the loss or absence of these secondary principles in human society - that is, circumstances can potentially “erase” these principles from a society. However, primary principles are universal and indelible.
This latter characteristic is the backbone of Natural Law: if we assume that all sufficiently rational creatures have access to a set of universal ethical principles, then we may also assume that, regardless of culture, we have the ability to reach objectively “True” ethical imperatives and prohibitions. Furthermore, we can critique culture from a standard that is outside or beyond culture: that is, our access to universal ethical precepts should allow us to criticize any culture, even our own. This is quite a powerful assumption, and it also carries with it the potential for a very specific kind of abuse not uncommon in history: if a person or peoples have a different set of ethical norms, we may assume that they are either wicked, in the sense of openly disobeying Natural Law, or rationally inferior, in the sense of being not developed enough to recognize it. There are positive elements to this assumption, because it would provide a common ground for any ethical discussion between cultures - more importantly, it suggests that there are universal ways to come to the “right” conclusion about ethics. However, I would argue that history has shown the misuse of Natural Law to be more harmful than its proper use has been beneficial. Genocide, religious war, and slavery have all often been justified by the dehumanization of its victims through arguments based on cultural diversity: more specifically, that the perpetrator or conqueror’s culture and ethical norms are superior to the victim, and that the victim is either hopelessly wicked, less-than-human due to their inferior intelligence, or both, and are thus deserving of death or slavery.
Of course, the affirmation of these claims - that a culture lacking the ethical principles of an assumed natural law is either wicked or inferior - would implicitly affirm that those principles can be lost, and thus deny that Natural Law is truly universal to humanity. That this conclusion is rarely arrived at by the conquerors or perpetrators of these acts is perhaps a sign that the theory has been misappropriated as a tool of political convenience.
Now, if the essential content of Natural Law really were universal, if follows that it would be present in the ethical principles of every human culture. Since secondary principles can be lost, they would not necessarily be present in every human culture. So, then, if there are such universal ethical principles, at least one way (and possibly the only way) to verify them is to conduct empirical investigations into every human society, both past and present, and see if such principles are found universally. After all, if empirical investigation found absolutely no such universal principles, then at least one element of Natural Law would be falsified: the concept that its principles are universal to rational creatures.
Thus follows my conclusion that empirical observation would be essential not only to demonstrating the truth of Natural Law but also to the description of its primary, indelible principles. But here lies yet another problem for Natural Law: how does one describe these principles? The more specific and substantial one's description of the principles, the more likely one will find ample evidence of cultures in which ethical norms lack or contradict one or more of those principles. For example, one would probably find far more cultures contradicting a principle such as “marriage is between one man and one woman” than would find contradicting a principle such as “marriage is between men and women.” Successively broader and more vague principles would likely have even fewer contradicting cultures. However, even one culture in which ethical norms contradicted or lacked such a principle would be enough to falsify the hypothesis that that principle is universal!
Now, such an undertaking would need certain sorts of caveats. For example, there are plenty of examples from history in which a society reacted to a sudden social ill or calamity in a way which is fundamentally at odds with its own ethical principles; such short-lived malaises may be excluded from the study if it represents only a momentary breach of a society’s underlying principles rather than a true change in principle. Researchers more learned and adept in these matters should set such requirements, but I would venture to guess that if such a breech in principles lasts less than the average lifetime of its members, it can be discarded, but that changes that last over more than one lifetime are indicative of a substantial and accepted change in that society’s ethical norms, even if later events return that society to its original (or nearly original) norms.
It is my hypothesis, simply from my rather scant knowledge of anthropology, ethnography and sociology that no substantial ethical principles will pass such an empirical test. In order to fashion an ethical principle that can be found in every culture, one would have to resort to a statement such as, “one should pursue good and avoid evil,” which is a fundamental tautology, since ethical good may be defined as “that which should be pursued” and ethical evil may be defined as “that which should be avoided.”
What would be the consequences of the discovery that no non-tautological ethical principles are truly universal? For one, Natural Law would lose its ability to provide critiques of cultural norms from “outside culture,” because it would be revealed that ethical principles are not universal but are dependent upon culture. I do not believe this conclusion to be ultimately irreconcilable with Christianity (which, in my mind, has always been extremely insistent upon the particularity - even the cultural particularity - of its revelation) nor even to cross-cultural criticism. Instead, it would destroy any attempt at moralizing from the pure ethical high-ground of universal law and would instead force all ethical conversations to proceed in culturally specific terms grounded in history and particular societies. None of this would either deny or affirm the presence of some “right answer” to ethical questions: it would simply change the terms (and perhaps, to some degree, the attitudes) of those undertake the discussion.