Cogito, Ergo Sum – Opinio Juris in an Ever-Evolving Landscape

Customary International law is not written in stone but etched in orbit—shaped by movement, belief, and time. (Photo: Emma Angel, Unisphere, NYC (Unsplash))

This article critically evaluates the concept of opinio juris sive necessitatis (opinio juris), examines its significance in the formation of customary international law (CIL), and explores its implications for the validity and functionality of customary rules. For the purpose of this analysis, a legal realist-influenced perspective will be employed to determine whether the inherently uncertain nature of opinio juris adversely impacts the establishment of customary rules or, conversely, influences the manner in which these rules are employed and acknowledged by various states and their representatives. To achieve this objective, the structure of CIL and the function of opinio juris within its development will be scrutinised. Furthermore, two principal challenges pertinent to the concept and functionality of opinio juris will be identified and analyzed, with the intention of assessing whether these challenges indeed hinder the evolution of CIL or influence associated elements.

Customary International Law

CIL is generally understood to encompass two essential elements: state practice and opinio juris. State practice refers to the general and consistent conduct demonstrated by states, while opinio juris signifies that this practice is acknowledged as law. These foundational elements have been recognised by the International Law Commission (ILC) in its Conclusions on the Identification of Customary International Law, where it asserts: “To 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).”

The application of this test by the International Court of Justice (ICJ) has demonstrated a notable inconsistency. In some cases, the lack of evidence supporting opinio juris leads to the conclusion that a claimed rule of CIL does not exist. Conversely, there are instances where, even in the absence of clear evidence of opinio juris from the states involved, the Court still determines that the asserted customary rule is valid. Additionally, there are also known situations in which the Court has acknowledged the existence of a customary rule based solely on evidence of opinio juris. This inconsistency may stem from the inherent uncertainty surrounding the nature, manifestation, and external recognition of opinio juris.

Opinio Juris

The fundamental concept of opinio juris can be elucidated through an idealistic interpretation of Descartes' principle, "cogito, ergo sum." This analogy posits that the legal significance attributed to a particular course of conduct imparts its legal character. One could argue that opinio juris is not an absolute truth akin to cogito, but rather a criterion that differentiates binding customs from mere practices. However, one interpretation of opinio juris posits that the subjective recognition and articulation of the legal significance of conduct reveal its legal nature. As noted by Walden, both the belief that a practice is mandated by law (regardless of its correctness—opinio juris) and the belief that such practice ought to be legally binding (opinio necessitatis) are critical components of custom and its validity.

The existence of opinio juris is a prerequisite for the validity of CIL, and this validity is a fundamental aspect of the development of international customary law. In its absence, conduct is merely a general practice rather than being recognized as a legally binding custom. As emphasized by Thirlway, "only if the view that the custom should be law has the effect of making it law (provided it is coupled with sufficiently general usage), can subsequent practice be coupled with the correct view that the custom is law.” Lowe asserts that opinio juris serves as the subjective element that elevates customary behaviour to the status of law, reflecting a sense of legal obligation or necessity. Consequently, Lowe's assertion that opinio juris embodies a belief held by states regarding the legal character of their actions complements Thirlway's understanding of the transformative power of such a belief.

The Challenge of Identifying and Expressing Opinio Juris

The existence of opinio juris within CIL introduces a sense of externality, ensuring that the state’s intention was to align its practices with CIL. This raises the question of how to accurately identify opinio juris. As a concept, opinio juris is subject to considerable debate: it is acknowledged that while it plays a pivotal role in transforming state practice into CIL, there is a lack of consensus among international lawyers regarding its nature and many of the theoretical challenges associated with it.

Lowe further highlights the complexity of identifying opinio juris, as it often requires interpreting state declarations, diplomatic correspondence, and official statements to discern whether a state perceives its actions as legally binding. His interpretation aligns with the traditional view that international law is dependent on the consent and recognition of states, with opinio juris serving as a crucial element in establishing the binding nature of customary practices. This concept supports the evolution and adaptation of international law, reflecting the collective legal conscience of the international community of states while balancing state sovereignty with communal norms. However, to demonstrate the existence of a rule of CIL, it is essential to establish that states act in a certain way because they recognize a legal obligation to do so. In the absence of explicit declarations regarding this obligation, it appears that opinio juris may be inferred from the consistent repetition of a particular course of conduct.

As articulated by Parry,"if a course of conduct is repeatedly followed, the only presumption, or at least a fair presumption, is that it was so followed, and another course not followed, due to the existence of a conviction of obligation." However, the International Court of Justice (ICJ) in the North Sea Continental Shelf case (para. 77) asserted that "the frequency, or even habitual character of the acts is not in itself enough." This leads to the question of how a state might effectively demonstrate opinio juris, given that a sole course of conduct may not be definitive enough.

Furthermore, the expression of opinio juris can exhibit considerable variation in its nature. Traditionally, opinio juris is derived from a state's expressions; however, the way a state articulates its position, whether in support of or in divergence from purported international legal norms, is often ambiguous. Such expressions may arise from declarations issued by senior government officials, formal statements presented in international forums, such as the United Nations General Assembly, or through endorsements of specific resolutions, including the Declaration on Principles of International Law concerning Friendly Relations and Cooperation among States in Accordance with the Charter of the United Nations. The co-existence of these expressions, often without contradiction, complicates their categorization as definitive evidence of legal necessity or legal obligations.

Evidence of opinio juris may take the form of diplomatic statements, voting patterns, or articulated views. However, in practice, such conduct may often be driven by political expediency rather than an obligation under the law or a commitment to legal principles. Müllerson contends that “state practice always includes both elements—objective and subjective,” yet the subjective component does not consistently meet the threshold required for opinio juris. This raises a broader concern regarding the inherent uncertainty of opinio juris and its critical role in CIL, thereby calling into question the validity of CIL itself, unless it is documented in written form.

This concern has led to initiatives such as the study conducted by the International Committee of the Red Cross (ICRC) on Customary International Humanitarian Law, which seeks to codify CIL and alleviate the uncertainty associated with the identification of unwritten opinio juris. Such documentation offers clarity in instances where unwritten custom may otherwise be too ambiguous to confirm.

Opinio Juris and the Evolution of CIL in a Changing International Landscape

The question of whether this uncertainty should be viewed as a problem remains open to interpretation. The fluidity of contemporary international relations adds a layer of complexity to the role of opinio juris in the formation of CIL. As global priorities shift and the influence of non-state actors grows, the traditional understanding of opinio juris becomes increasingly difficult to apply consistently. Emerging challenges, such as environmental protection and digital rights, demand adaptable norms. The pursuit of a rigid and precise definition of opinio juris may impede the timely development of new customary laws in these areas.

Ultimately, this uncertainty allows for flexibility, which is essential for the international community to respond effectively to diverse situations. Given the inherently uncertain nature of law, achieving certainty remains a significant issue, regardless of how certainty is defined or whether it is considered desirable. This flexible aspect of the law arises from its close connection to society, which is itself inherently inconsistent. Legal realists, such as Cardozo, argue that the law is not constrained by a definitive set of abstract principles or rules; instead, it is shaped by factors like judicial decisions, social and economic trends, and power dynamics. Cardozo’s reflections on ambiguity and the interpretive role of judges can be applied to the fluid nature of CIL and the challenges presented by opinio juris. His ideas about judicial creativity and the necessity for adaptation within legal systems resonate with ongoing discussions in international law, particularly concerning the evolution of customary norms. Cardozo’s perspective underscores how legal standards, including those at the international level, may develop over time, necessitating that judges and practitioners adapt to new circumstances and evolving norms, often amid significant uncertainty and thus welcoming the uncertain nature of opinio juris.

Conclusion

This article has critically examined the nature, manifestation, and recognition of opinio juris. The analysis indicates that the identified challenges, which impact both individual states and the international community collectively, do not undermine the formation of CIL per se, but rather affect the recognition and utilization of customary rules. Significantly, external perspectives on opinio juris do not inherently affect its validity. As such, the evolution of customary law remains intact; what is altered is how these norms are interpreted and acknowledged by various actors within the international legal system.

Melis Irem Kirkdisceoglu (21) is an international student from Germany currently pursuing an LLB at the University of Nottingham. Her primary academic interests lie in public international law, particularly international investment law, legal theory, and the evolving relationship between law and societal development. As an editor of The Nottingham Advocate and an active contributor to the European Society of International Law (ESIL) and the International Law Association (ILA), she engages critically with international legal debates, with a focus on systemic reform and legal innovation. Melis is committed to advancing perspectives that reflect the law’s changing role in global governance.