Between theory, practice, and ‘interpretation’ of customary international law
by Michael Wood and Omri Sender
Published on November 2022
Customary international law remains a principal source of international law, and it comes therefore as no surprise that it continues to capture the attention—and imagination—of scholars and practitioners alike. The recently published volume entitled The Theory, Practice, and Interpretation of Customary International Law contains some thoughtful essays on questions both old and new, seeking to add to a scholarly discussion on customary international law that spans centuries of legal thought. This longstanding intellectual effort, as Kadens and Young have shown elsewhere, has grappled with theoretical difficulties that indeed ‘have been around for more than half a millennium.’ It has also demonstrated, they observe, that ‘[t]he prospects for international custom must be assessed in light of actual experience, not just theory.’
In suggesting—perhaps somewhat defensively—that ‘we need more theory, not less’, two of the book’s editors explain that even though the International Law Commission (ILC) ‘presented the world with an authoritative reading of CIL “theory”’ when adopting in 2018 its conclusions on ‘Identification of customary international law’, that could not have put to rest ‘genuinely legal-theoretical and legal-philosophical questions, … e.g. “is the creation of CIL itself regulated by legal rules”.’ Among other ‘questions relevant to the theoretical problems of CIL’, they refer to the current health of the two-element approach to identifying rules of customary international law, the possibility of interpreting such rules, and the approaches taken by various fora in regard of these matters.
Are these questions really only theoretical? Can they fully or usefully be considered without assessing how customary international law actually operates in practice? The book itself appears to suggest that they cannot, not least because several chapters in the part devoted to ‘The theory of customary international law’ in fact look to the ‘real world’ in validating their claims. Arajärvi’s chapter, for instance, combines ‘doctrinal study with empirical examples’ in exploring the question of ‘whether there are detectable objective standards for the determination of misinterpretation [of customary international law] or whether such observation is a subjective one.’ Baka’s chapter, likewise, looks to the decisions of the International Court of Justice in evaluating the significance, for customary international law, of ‘absence’ embodied by State silence. The actual conduct and interests of States are central to the thesis proposed in Beham’s chapter too.
This is just as well, for international law is best appreciated when theory is not divorced from practice. That is not to go so far as to say, with Goethe’s Faust, that –
Grau, teurer Freund, ist alle Theorie,
Und grün des Lebens goldner Baum.
Purely doctrinal questions no doubt have their place, especially where their study affords the law a more secure foundation, and where the practice of international law may sometimes be enriched by theory. The ILC’s work on ‘Identification of customary international law’ does not suggest otherwise, as may readily be seen from the extensive bibliography prepared by the Special Rapporteur on the topic. All the same, the conclusions do stand in contrast to the view that customary international law is an ‘inherently theoretical topic.’ To borrow from Wittgenstein’s depiction of philosophy, the determination of customary international law is not a theory but an activity.
One other question that the book examines from both a theoretical and a practical perspective is whether customary international law can be ‘interpreted.’ That depends on what is meant by ‘interpretation.’ If all that is meant is determining the content of a rule of customary international law, then the ILC’s answer is given in its conclusion 2, which states that ‘[t]o determine the existence and content of a rule of customary international law, it is necessary to ascertain whether there is a general practice that is accepted as law (opinio juris).’ Giving any other meaning to ‘interpretation’ might result in unsupported conclusions as to the content of the customary rule in question, for example, by introducing an excessive measure of deduction. Indeed, it does not at all ‘seem obvious to wish to interpret [customary international law]’: the notion of interpretation, which is commonly employed in the context of giving meaning to a text, appears intrinsically inapplicable to the unwritten rules of customary international law. Judicial decisions that have used the term in its Vienna Convention on the Law of Treaties meaning seem to concur: for example, the Special Supreme Court of Greece stated in Margellos and Others v. Federal Republic of Germany that it could not derive a customary rule through interpretation but must infer its existence by invoking specific evidence of international practice. The Tribunal in Glamis Gold, Ltd. v. United States of America similarly ‘emphasize[d] that the task of seeking the meaning of “fair and equitable treatment” by way of treaty interpretation is fundamentally different from the task of ascertaining the content of custom [on that matter].’
This is not to say that the state practice and evidence of opinio juris underlying a rule of customary international law cannot themselves be interpreted; they may well need to be, so as to evaluate their relevance and significance. For instance, action (or inaction) by a State may sometimes have to be interpreted so as to ascertain to what degree it constitutes acceptance as law. Interpretation may also be called for in the case of written materials that are resorted to determine a rule of customary international law, such as treaty provisions, resolutions of international organizations or conferences, judicial decisions, ILC texts and even writings, although any such interpretation would be of such textual formulations and not of the customary rule as such.
The question whether the identification of customary international law is itself regulated by legal ‘rules’, too, clearly has a practical significance. The ILC avoided entering into a debate about the nature of its conclusions: whether, for example, they are ‘rules of recognition’ for customary international law, or ‘meta-law’ to be traced back to some Grundnorm. Even so, international law surely prescribes how a rule of customary international law may come into being, and the ILC did endorse, when taking up the topic, the view according to which ‘the Commission should aim to describe the current state of international law on the formation and evidence of rules of customary international law’ (emphasis added). The basic approach enshrined in the conclusions was thus widely considered to reflect the existing position under international law, without ‘progressive development’: conclusion 1 (and the general commentary) make it clear that the conclusions concern ‘the way in which the existence and content of rules of customary international law are to be determined’ (emphasis added). As one member of the Commission observed in taking stock of the work on the topic, the conclusions ‘rest on a firm basis in international law and practice and … are therefore not merely recommendatory.’