Introduction
Customary International Law (CIL) is a fundamental source of international law, recognised by Article 38(1)(b) of the Statute of the International Court of Justice as evidence of "a general practice accepted as law". The statement in question suggests that this definition is founded on the principle that the consent or acceptance of states is the basis for creating legally binding rules of general application. This essay will critically discuss this statement. It will argue that while the traditional theory of CIL is rooted in the concept of state consent, this view is an oversimplification. The actual process of customary law formation, including the requirements for state practice and the persistent objector doctrine, reveals that CIL operates on a more complex basis than simple consent, often binding states through acquiescence or their membership in the international community, rather than their express agreement.
The Consensual Basis of Customary Law
The classic positivist or 'voluntarist' theory of international law posits that its rules derive their authority from the will of sovereign states. States are only bound by rules to which they have, in some form, consented. This perspective was famously articulated by the Permanent Court of International Justice in the S.S. Lotus case, which stated that "rules of law binding upon States… emanate from their own free will" (PCIJ, 1927, p. 18). From this viewpoint, the two constituent elements of CIL—state practice and opinio juris—are direct evidence of this consent. State practice represents the material acts of states, demonstrating their conduct and will. Opinio juris, the belief that this practice is followed out of a sense of legal obligation, confirms that the conduct is not merely habitual but is intended to create or affirm a legal rule. In this model, CIL is essentially a form of tacit treaty; states, through their consistent behaviour and belief, implicitly agree to be bound by a rule. This consent-based framework gives CIL its legitimacy, as it respects the sovereignty of states.
Challenges to the Consent Model
Despite the theoretical appeal of the consent model, it faces significant challenges when applied to the practical realities of CIL formation. The International Court of Justice (ICJ) in the North Sea Continental Shelf cases clarified that for a rule of CIL to emerge, state practice must be "both extensive and virtually uniform" but not necessarily universal (ICJ, 1969, para. 74). The term 'general' practice, as opposed to unanimous practice, implies that a rule can become binding without every state having actively participated in its formation. The practice of a minority of states, particularly those most powerful or specially affected, can be sufficient to create a rule that then applies to all. This immediately raises a problem for the consent theory: how can states that did not participate in the practice, or even existed at the time of the rule's formation (i.e., new states), be said to have consented? The answer is that they cannot, at least not in any explicit sense. Their obligation appears to arise automatically from their membership in the international community, a concept at odds with strict voluntarism.
The Role of Acquiescence and the Persistent Objector
The consent theory is further qualified by the doctrine of the persistent objector. As indicated in the Anglo-Norwegian Fisheries case, a state that has "always opposed any attempt to apply" a customary rule to its coast will not be bound by it (ICJ, 1951, p. 131). At first glance, this appears to support the consent model by allowing a state to opt out. However, the doctrine's existence implies a crucial corollary: a state that fails to object persistently from the rule's inception is deemed to have acquiesced and will be bound. This shifts the basis of obligation from active consent to a failure to dissent. This 'tacit consent' is a legal fiction, as inaction may occur for many reasons other than agreement, such as a lack of capacity or awareness (Shaw, 2021). Therefore, the general applicability of CIL seems to stem not from universal positive consent, but from a general lack of opposition. Once established, the rule applies to all states except the few who can prove their status as persistent objectors. This suggests a legislative-style process where a majority or influential group of states creates law for the whole community, a departure from the idea that each state individually consents to every rule that binds it.
Conclusion
In conclusion, the statement that the definition of custom is essentially a statement of the principle of state consent is only partially accurate. While the language of consent and acceptance underpins the traditional theory of CIL and provides it with legitimacy, it does not fully capture the dynamic of its creation and application. The requirements of 'general' rather than universal practice, the binding of new states, and the principle of tacit consent through acquiescence all demonstrate that states can be bound by rules to which they have not explicitly agreed. The consent of the state, in practice, has been replaced by the consensus of the international community. The definition of custom in Article 38(1)(b) is therefore more than a simple reflection of individual state consent; it is a formula that balances state will with the need for a stable and universally applicable legal order.
References
- Anglo-Norwegian Fisheries Case (United Kingdom v Norway) (1951) ICJ Rep 116.
- North Sea Continental Shelf Cases (Federal Republic of Germany v Denmark; Federal Republic of Germany v Netherlands) (1969) ICJ Rep 3.
- Shaw, M.N. (2021) International Law. 9th edn. Cambridge: Cambridge University Press.
- Statute of the International Court of Justice (1945) 1 UNTS XVI.
- The Case of the S.S. "Lotus" (France v Turkey) (1927) PCIJ Rep Series A No 10.
