Document Type : Research Paper

Author

• Mahdi Haddadi

Associate Professor, Department of Public International Law, Faculty of Law, Collage of Farabi, University of Tehran, Qom, Iran

Abstract

1. Introduction
Several centuries have passed since the emergence of the international law system. Over time, the development of international rules has increasingly influenced and regulated numerous aspects of human activity. The expansion of these rules, along with the proliferation of international courts, makes it essential to clarify how the rules of international law should be interpreted and applied. In this growing international legal landscape, where customary international law continues to serve as one of the primary sources of international law, little attention has been given to the question of whether it is possible to interpret the rules of customary international law. Throughout history, customary international law has been the subject of extensive research. Studies on the topic continue even within international institutions. For instance, the International Law Society conducted a 16-year project (1984–2000) on the formation of customary international law. Similarly, the International Law Commission (ILC) began a study in 2013 on the recognition of customary international law, completing it in 2018. Although the ILC produced valuable findings—particularly by noting that determining the existence of a customary rule and its content are simultaneous processes—it did not independently address the question of how to determine the content of customary international law or the related issue of its interpretation. In other words, once an unwritten rule is identified, can it then be meaningfully interpreted? Other ILC work that touches on interpretation—such as studies on the fragmentation of international law or subsequent agreements and procedures in treaty interpretation—has never fully explored the intersection of interpretation and customary international law.
Although extensive research has been conducted in the areas of treaty interpretation and the formation and recognition of customary international law, the point of convergence between these two fields—namely, the interpretation of customary international law—remains largely unexplored. The almost exclusive focus on the formation of customary international law (considering its two elements of state practice and opinio juris) may help explain this gap. Some jurists, for instance, maintain that customary rules cannot be interpreted, and that only written texts are subject to interpretation. Others take the position that while the rules of customary international law can be identified, they cannot be interpreted. There are two main arguments regarding the interpretability of the rules of customary international law. First, it is argued that interpretation applies only to written rules, and since customary rules are unwritten, they cannot be interpreted. Second, it is claimed that the identification of customary rules is the sole process that determines their content; consequently, any attempt to explain a customary rule would require a new stage of identification. The present study aimed to examine the validity of these arguments by analyzing opinions from both the doctrinal sources and judicial jurisprudence concerning the interpretability of customary international law. The study went on to address the methods by which customary rules can be interpreted, should their interpretability be established.

Literature Review

No research on this topic exists in Persian, and studies in other languages remain limited. The current inquiry reviewed the main literature on the subject.

Materials and Methods

The present study used a descriptive–analytical method to examine the validity of the arguments about interpretability of the rules of international law. By examining both doctrinal sources and judicial jurisprudence, it ultimately presented the methods for interpreting customary rules.

Results and Discussion

The analysis focused on whether customary rules can be interpreted and, if so, how such interpretation should be carried out. There is no consensus among international law scholars on this issue. Some argue that the development of customary international law involves the intertwined processes of identification, interpretation, and enforcement. Others maintain that customary international law cannot be interpreted because it is unwritten. Despite the challenges and ambiguities associated with custom, its status as a source of international law is unquestioned. A logical question therefore arises: once a customary rule has formed—and aside from the possibility of its modification or termination through subsequent practice and opinio juris—does its inherent ambiguity not necessitate interpretation for its continued application? The very existence of ambiguity and uncertainty—even in written instruments—is what makes interpretation necessary. It is thus counterintuitive to deny the interpretability of norms derived from a source of law that shares these same characteristics. Interpretation, which involves determining the meaning of a legal act, is equally applicable to both written and unwritten rules. In interpreting the law, we interpret social facts, which may be expressed verbally or nonverbally. Moreover, all laws, whether written or unwritten, leave room for uncertainty and therefore for interpretation.
In response to the claim that interpreting customary international law can be reduced to identification, it is important to distinguish between rule identification and content determination. Identifying a rule is the process by which a legal norm is recognized, whereas determining its content involves defining the meaning of that norm. Although these two processes are often intertwined and complementary in practice, they remain conceptually distinct. Regarding the method of interpretation of customary rules, it was clarified that although Regarding the methods for interpreting customary rules, although Articles 31 and 32 of the Vienna Convention on the Law of Treaties apply only to treaty interpretation, the interpretive techniques they contain have acquired a customary character and have been used in various contexts to interpret acts that are not treaties. They are employed, for example, in interpreting unilateral acts, declarations accepting the jurisdiction of the International Court of Justice, and Security Council resolutions. What these cases have in common is that Articles 31 and 32 are not applied strictly; rather, they are applied mutatis mutandis, with due regard for the specific nature of the acts being interpreted. This approach appears equally appropriate for the interpretation of customary rules.

Conclusion

It can be concluded that customary international law, like written law, requires interpretation due to its inherently general and often ambiguous nature. The distinction between identifying a rule and determining its content underscores that interpretation is an independent and indispensable process. Moreover, the interpretative methods developed in treaty law, when appropriately adapted, can also be applied to customary norms. Accordingly, denying the
interpretability of customary law is neither theoretically convincing nor practically viable, whereas acknowledging it contributes to greater coherence and effectiveness within the international legal system.

Keywords

• Custom
• Treaty
• Interpretation
• Identification of Custom
• Methods of Interpretation

Main Subjects

• International Law