Customary Law Interpretation as a Tool: of Concretization and Evolution

Customary international law (CIL) is one of the three primary sources of international law. Historically, CIL is considered one of the cornerstones of the international legal order, and the process through which some of the foundational rules of international law have come into existence. In scenarios where various international legal regimes might conflict or intersect, customary rules can provide a proverbial least common denominator of the legal obligations in play. Moreover, since CIL rules are the organic product of the interactions that take place within the international legal system, they are capable of evolving alongside the community in which they operate. In this sense, CIL is a Janus-like source, not in the derogatory sense of the term, but rather in the original sense attributed to the Roman deity. A source that has one face looking in the past where some of the oldest rules that regulate international law emerged, and another one looking forward to emerging rules, that are developed as state conduct evolves.
The Janus analogy could be pressed even further by referring to the duality of elements required for the emergence of CIL rules ‘as evidence of a general practice accepted as law’. This formulation, enshrined in Article 38 of the Statute of the Permanent Court of International Justice and copied verbatim to that of the International Court of Justice (ICJ), has been taken to imply that for a customary rule to form two constitutive elements need to be present – state practice and opinio juris. The customary rule is then identified through an inductive evaluation of these two. While CIL identification has been widely studied, relatively less attention has been paid to what happens to a CIL rule once it has been identified. In its recent work on the subject, the International Law Commission (ILC) seems to have accepted that there can be a distinction between a CIL rule’s existence and its content-determination, a point raised more forcefully by the Netherlands in its comments to the ILC. In the ILC’s view this was especially visible when ‘in some cases it is accepted that the rule exists but its precise content is disputed’. Nevertheless, the ILC remained silent on what this distinction entails or implies. Similarly, while acknowledging that customary rules may change or evolve over time, it excluded further consideration of these issues from its inquiry.
We depart from the premise that what happens in the aftermath of the identification of the existence of a CIL rule is interpretation. Once a CIL rule is identified through an analysis of state practice and opinio juris, its existence is not a static one but a continuous one. The content-determination of that rule now becomes a matter of interpretation within the given legal and factual contexts of the situations to which it may be called to apply. In fact, recent scholarly engagement with the subject demonstrates that interpretation is ubiquitous among courts engaging with customary rules, and that interpretation does in fact contribute to the continued existence and evolution of customary rules. Taken together, these considerations beg a foundational question – What is interpretation in the context of CIL, and what role does it play?
This Reflection introduces a broader thematic series hosted by ESIL Reflections on The Role of Interpretation in the Practice of Customary International Law. The series addresses the topic of CIL interpretation from both a practice-oriented and a theoretical perspective. What unites the different Reflections of the series is their analytical lens, namely, ‘interpretation as a tool’. ‘Tool’ in this context reflects the instrumentality of interpretation, and implies an active understanding of interpretation as entailing not only the concretization of the content of a rule, but also allowing for its evolution. This captures the multifaceted nature of interpretation as an operation in the context of CIL. The thematic series thus maps the various roles that interpretation plays in the ‘life-cycle’ of customary rules.
In this Reflection, we discuss the analytical framework of the series in more detail, and introduce the way we conceptualize interpretation when it comes to CIL. We argue that interpretation plays two functions in the continued existence of CIL rules: a concretizing/collapsing and an evolutive function. In this sense, interpretation can be both a tool for the solidification of customary rules and a tool for their maintenance and ‘updating’, without each function being prohibitive of the other. We begin this discussion with a brief description of the way we conceptualize interpretation and the type of interpreter we have in mind. We then delve into the two functions of interpretation in the practice of CIL, and discuss the aspects which make interpretation a ‘tool’. It is here that we also introduce the contributions of the other authors of the special series. Finally, we bring all the arguments together by way of conclusion.