Customary International Law Symposium: Back to the Guillotine – Namely Hume’s Guillotine

[Martin Scheinin is a Professor of International Law and Human Rights at European University Institute and a former UN Special Rapporteur on Human Rights and Counter-terrorism.]

Professor Monica Hakimi’s article ’Making sense of customary international law’ is both rewarding and thought-provoking. It fully merits this Symposium. She makes a convincing case that most if not all mainstream doctrinal writing on the topic has serious flaws. She rightly criticizes what she calls the “rulebook conception” of customary international law and convincingly demonstrates that in everyday practice it does not really work like a rulebook. Also, she is right in emphasizing that law also is a social phenomenon.

While I agree about the diagnosis, I am not convinced that Hakimi has found the underlying cause of the patient’s illness. Instead, I tend to believe that behind the problem identified by Hakimi, but also inherent in and burdening her answer, is a common underlying and fundamental problem. That problem is the constant confusion between the sources and norms of international law, particularly between custom and customary norms of law. Sources are facts, belonging to the world of Sein/Is, while norms are commands, belonging to the world of Sollen/Ought, as taught to us by David Hume and Immanuel Kant. The problem is deeper than merely related to customary international law. It is about law as a normative phenomenon detectable only through social practice.

Article 38 of the Statute of the International Court of Justice is routinely referred to as a provision about the sources of international law, even if its wording does not even mention the term “source”. The Court “shall apply”, inter alia, “international custom, as evidence of a general practice accepted as law” (Article 38.1.b). The confusion between norms and sources is thereby built into the provision itself. Custom as such is not a norm but a fact. What transforms (still as a fact) it into a source of law, of binding legal norms, is its acceptance (again a fact) as such, i.e. as legally binding. As H.L:A. Hart would say, we have a rule of recognition (a norm) here, albeit expressed in awkward terms: the factual pattern of constant and more or less consistent practice of States becomes a habit, and if this process also represents the acceptance of such a consistent practice having been accepted as a legal obligation of States, then we will recognize that the legal order of international law contains a valid legal norm that States have an obligation to follow that practice. As Article 38 is not an all-purpose provision about sources of law in a World Constitution, but technically addresses only the International Court of Justice, it resolves the question of sources for purposes of the operation of that Court. The rule of recognition expressed in Article 38.1.b replaces the need for States positing a norm through a treaty.

In part analogously, Article 38.1.a refers to “conventions” that have factual existence, and to their express recognition by States as another fact. These two facts are defined as the basis for a rule of recognition that the legal order of international law accepts treaties between States as a source of binding legal norms, for the purpose of being applied by the International Court of Justice. Correspondingly Article 38.1.c recognizes as norms of international law “the general principles of law recognized by civilized nations”, allowing for the conversion of legally binding norms of domestic legal systems, the existence of which for purposes of international law is an external fact (a source of law), to legally binding norms within the legal system of international law and again for application by the Court.

Hakimi is right about many things. Yes, international law also is a social phenomenon that belongs to the world of Sein/Is, i.e. is characterized by its institutionalized existence and social practices. And yes, customary norms of international law are a different category of legal norms than treaty norms because of the absence of one central authority in the creation or emergence of such norms and a higher degree of ambiguity as to the exact relationship between observable formulations of the norm and the underlying legal norm. Whoever applies a norm of customary international law needs first to formulate a proposition about that norm, even if the source of the norm has not been posited in a treaty. Once an authoritative body – such as the International Court of Justice – has formulated and applied a norm of customary international law, others can either rely on that body as an authority and use the same formulation, or they may seek to contribute to the process of the evolution of customary international law through presenting a new formulation of the underlying norm.  Hakimi is right, the process lacks a definitive “end point”. But this is not fundamentally different from how treaties operate as a source. What makes the application of treaties as a source easier, is that they have been posited by States, with the entry into force of a treaty as a defined “end point”, and their status as authority therefore is more clear. Nevertheless, every interpreter of a treaty will add new layers into the understanding of what exactly is the legal norm expressed in the text. Because of their unwritten nature, however, customary norms are more likely to be subject to mutually incompatible competing claims of validity than treaty norms where disputes focus on interpretation. As Hans Kelsen explains, while a legal order mainly consists of norms posited on the basis of authority provided by the same legal order, the ultimate foundational norm (the Grundnorm) is merely presumed rather than posited, and its source is factual power. For the rule of recognition to function, it is important to have clarity about by whom new norms may be posited (treaty-based norms) or articulated (customary norms).

Custom and treaties are facts, sources of law. They belong to the world of Sein/Is, while customary norms and treaty norms belong to the world if Sollen/Ought. The latter world is characterized by validity, not existence. This characteristic defines the legal order of international law as a normative phenomenon. Realists are wrong in trying to reduce law merely to a social phenomenon, be they apologists who reduce international law to what States actually do and cherish it as such, or nihilist critics who reject law as a mere extension of politics. Law, including international law, is a normative order with a quest for coherence.

The practice of international law certainly is, as Hakimi writes, a “real-world sociological phenomenon”: a practice, i.e. a constant flow of patterns and chains of facts. Observing this social phenomenon will help us to understand what sources (still facts) are, through that same social practice, recognized as evidence of legally binding norms. There is no shortcut for deriving norms directly from facts. It is only the factual phenomenon of social practice that will tell us what are the norms that have been derived from the sources and recognized as legally binding rules, thereby constituting the valid normative order of international law.