The Role of Natural Law as a Source For International Law

Within the context of our roles as part of a research group at Princeton’s Center for Theological Inquiry, Mary Ellen and I have had many wonderful conversations about natural law as a source for international law. My sense is we both share the view that natural law could be such a source, and we have discussed various instances in which that might occur. Let me briefly summarize the three most obvious possibilities.

First, natural law might be relevant to identify and define jus cogens norms. Jus cogens, of course, is important as a potential limitation on positive law. Mary Ellen suggests that international courts can serve as the proper forum for deciding what constitutes a jus cogens norm. But there is a key problem with using international courts: the ability of international courts to opine about substantive jus cogens norms is subject to jurisdictional limitations. International courts are rarely be vested with authority to adjudicate questions of jus cogens obligations, particularly of the non-positive law variety. The one great exception is an Advisory Opinion by the International Court of Justice. But even then—as with the advisory opinion on the Legality of Nuclear Weapons—the ICJ relies almost exclusively on positive law to resolve the questions presented. The voice of international courts is authoritative, but they do not have the jurisdictional authority.

Second, natural law might be relevant to identify and define crimes that justify national court assertion of universal jurisdiction. In this context, national courts can adjudicate questions of what sort of crimes are so heinous that any national court can assert jurisdiction over the person. It would seem that recourse to sources beyond positive law would be available to address this question. The Eichmann case is the archetype. But, of course, universal jurisdiction is extraordinarily controversial because it vests authority in national court judges to make decisions on matters of supreme international import that they are ill-equipped to decide. National courts have the jurisdictional authority, but their voice is not authoritative.

Third, natural law might be relevant to bolster arguments for the establishment of positive international law. One can think of numerous international human rights treaties that were enacted based on an appeal to moral authority rather than state interest. If one canvasses the history of international human rights law, for example, many of the arguments rested on notions of natural law. The typical refrain is “Civilized people simply do not engage in this sort of behavior. Consequently, we should enact a treaty that will secure near universal adherence to this norm as a reflection of this commitment.” In this sense, natural law is instantiated in positive law. But once it becomes positive law, we tend to ignore the natural law underpinnings that support and justify the positive obligation. But the legislative authority of natural law is significant, even if its adjudicative or executive authority is not.