---

Might It Just Work? How To Interpret Customary International Law (Maybe)

By Dr Massimo Lando
Published on 6 March 2023

---

Green light, but only in principle

The feasibility of interpreting customary international law has been a matter of much academic debate. Essentially, this debate concerns whether customary rules can be objects of interpretation, which is a process to find out what they mean, as distinct from identification, which is the process to ascertain their existence.

Academic opinion is divided. Some argue that it is feasible to interpret customary rules, many of whom congregate around the TRICI-Law Project (e.g., Chasapis Tassinis, Cohen, Gorobets and Merkouris). Others argue that the development of customary international law merges elements of identification, interpretation and application (e.g., Fahner). I have argued that identification, not interpretation, is the process to find out what those rules mean. The case for the interpretability of custom builds on the assumption that customary rules are identified by inductive reasoning based on evidence of state practice and opinio juris (Jurisdictional Immunities of the State, para. 55). It is on this assumption that the case fails to persuade because state practice and opinio juris are central both to identifying customary rules, and to finding out their meaning.

The proponents of the interpretability of custom have not explored how one may have to find out the meaning of customary rules where those rules have been identified by non-inductive reasoning. This possibility is a real, practical one. International courts and tribunals mostly use non-inductive reasoning to identify customary rules (e.g., Roberts and Talmon). In most cases, they infer the existence of new customary rules from the existence of others, established ones (e.g., Military and Paramilitary Activities, para. 202), or simply assert what customary international law is (e.g., Nicaragua v. Colombia, paras. 114-118). Non-inductive reasoning in custom identification does not rely on evidence of state practice and opinio juris. Identification thus is not enough to tell us the meaning of customary rules identified by non-inductive reasoning. To find out what those rules mean, it might just be possible to interpret them.

This possibility would remain purely theoretical unless there were means of interpretation that can govern the interpretive exercise. With no aspiration to exhaustiveness, there seem to be four main candidates, all of which raise difficulties.

Means of interpretation

The first means is resort to evidence of state practice and opinio juris. Consistency with the two-element theory of custom could suggest using this evidence to find out the meaning of customary rules identified by non-inductive reasoning. The problem of using evidence of state practice and opinio juris is that an interpretive exercise based on it would replicate custom identification by inductive reasoning. This replication would negate the autonomy of interpreting customary international law as a process separate from that to identify it.

The second means is the framing of a rule by the states parties to a case. When dealing with custom identification, international courts and tribunals do not frame the relevant rules themselves, but simply decide whether those rules, as framed by the parties, are part of customary international law (e.g., Burkina Faso/Mali, paras. 20-26). It would be for the parties to make submissions as to the meaning of new customary rules, while international courts and tribunals would only have to decide if rules having the meaning as submitted exist in customary law. Relying on the parties’ submissions raises three problems. First, this approach would limit the jurisdiction of international courts and tribunals to determine what the law is. Second, this approach would generalise the parties’ submissions because, once identified, customary rules would guide the conduct of all states, at least in principle. Third, this approach may appear respectful of state sovereignty but it is not: it is only the sovereignty of the parties that would be respected, while the generalisation of the parties’ submissions is all but respectful of the sovereignty of all other states.

The third means would require interpreting a new customary rule consistent with the meaning of the established one from which it was inferred. This means is insufficient to find out the full meaning of the new customary rule. At most, the meaning of the established customary rule can circumscribe the meaning of the new one, but cannot unequivocally determine it. There can be a range of potential meanings consistent with the meaning of the established customary rule. Take the customary rule defining the scope of the immunity of Foreign Ministers from the criminal jurisdiction of a foreign states (Arrest Warrant, paras. 51-55). The immunity of high-raking state officials flows from the immunity of the state itself, which derives from the principle of sovereign equality of states (Jurisdictional Immunities of the State, para. 57). The principle of sovereign equality says little of how broad the immunity of Foreign Ministers is or should be. It would be consistent with that principle if immunity were absolute. It would also be consistent with that principle if immunity did not extend to certain minor offences that do not carry custodial sentences, which would not impair a Foreign Minister’s discharge of their functions.

Consistent interpretation raises two additional problems. First, when international courts and tribunals infer customary rules from established ones, they generally do so based on the principle of sovereign equality of states (e.g., Military and Paramilitary Activities, para. 202; Jurisdictional Immunities of the State, para. 57; Certain Documents and Data, para. 27). One can trace all rules of international law back to the principle of sovereign equality of states. That principle can tells us everything and nothing as to the content of customary rules which international courts and tribunals may infer from it. Second, consistent interpretation could only work were customary rules are inferred from established ones, but not where, as it often takes place, international courts and tribunals only assert what customary international law is.

The fourth means of interpretation is teleological interpretation. Merkouris has written that this means of interpretation is the main one to find out the meaning of customary rules identified by inductive reasoning (here, pp. 142-154). In this respect, the difficulty of teleological interpretation is how to determine the telos of a rule (Lando, pp. 1055-1056). This difficulty also exists where rules are identified by non-inductive reasoning. For example, the customary rule on uti possidetis iuris can have various objectives: ensuring the stability of boundaries; avoiding tension arising from border disputes from escalating; or ensuring the smooth accession to independence of former non-self-governing territories. To determine the telos of the rule, which would inform its interpretation, one must choose among various options. How to do so is the question. For example, one can rely on evidence of state practice and opinio juris, which would provide the context in which a rule was formed and thus show the rationale for its formation. This reliance would result in replicating identification by inductive reasoning, thus removing the need for interpretation entirely. Otherwise, one could determine the telos of a rule by reference to the telos of the established rule from which it was inferred, but the same problems arise as in relation to consistent interpretation.

Making ‘in principle’ work

In principle, it is possible to interpret customary rules identified by non-inductive reasoning. The problems lie in the means of interpretation. In this post, I have ventured to consider some of such means of interpretation, but all of them pose conceptual or practical challenges that are not easily solved. Scholars studying the analytical feasibility of interpreting customary rules should interrogate whether there are any means that one can convincingly use to find out the meaning of  rules identified by non-inductive reasoning. Beyond the framework of state practice and opinio juris is where that inquiry is most promising.

---