Opinio juris, a term frequently used in legal proceedings, is a shortened form of the Latin phrase opinio juris sive necessitatis, which means “an opinion of law or necessity”. In customary international law (one component of international law, customary international law refers to international obligations arising from established international practices, as opposed to obligations arising from formal written conventions and treaties; customary international law results from a general and consistent practice of states that they follow from a sense of legal obligation), opinio juris is the second element necessary to establish a legally binding custom.
Opinio juris denotes a subjective obligation, a sense on behalf of a state that it is bound to the law in question. The International Court of Justice reflects this standard in ICJ Statute, Article 38 by reflecting that the custom to be applied must be “accepted as law”. As with customary international law, opinio juris is an unsettled and debated notion in international law. Opinio juris sive necessitatis (“an opinion of law or necessity”) or simply opinio juris (“an opinion of law”) is the belief that an action was carried out as a legal obligation.
Opinio juris sive necessitatis
In international law, opinio juris is the subjective element used to judge whether the practice of a state is due to a belief that it is legally obliged to do a particular act. When opinio juris exists and is consistent with nearly all state practice, customary international law emerges. Opinio juris essentially means that states must act in compliance with the norm not merely out of convenience, habit, coincidence, or political expediency, but rather out of a sense of legal obligation.
Because opinio juris refers to the psychological state of the state actor, it can be difficult to identify and to prove. In practice, a variety of sources tend to be used to demonstrate the existence of opinio juris, including evidence such as diplomatic correspondence, press releases and other government statements of policy, opinions of legal advisers, official manuals on legal questions, legislation, national and international judicial decisions, legal briefs endorsed by the state, a pattern of treaties ratified by the state that all include the same obligation(s), resolutions and declarations by the United Nations, and other sources.
OJ is described as the psychological component of customary international law because it refers to an attitude that states have toward a behavioural regularity. The idea of opinio juris is mysterious because the legal obligation is created by a state’s belief in the existence of the legal obligation. OJis really a conclusion about a practice’s status as international law; it does not explain how a widespread and uniform practice becomes law.
The International Court of Justice
The International Court of Justice (ICJ), sometimes called the World Court, is the principal judicial organ of the United Nations (UN). The ICJ’s primary functions are to settle international legal disputes submitted by states (contentious cases) and give advisory opinions on legal issues referred to it by the UN (advisory proceedings). Through its opinions and rulings, it serves as a source of international law.
Article 38 of the Statute of the International Court of Justice explains customary international law as comprising of “(1) a general practice (2) accepted as law”. The ICJ, in its jurisprudence, has relied on, and interpreted, Article 38 to include two elements that assist the Court to determine the existence of an alleged customary international law – state practice and opinio juris (also known as opinio juris sive necessitates). The ICJ explained opinio juris, in the Nicaragua case, as follows: “for a new customary rule to be formed, not only must the acts concerned amount to a settled practice, but they must be accompanied by opinio juris sive necessitatis. Either the States taking such action or other States in a position to react to it, must have behaved so that their conduct is evidence of a belief that the practice is rendered obligatory by the existence of a rule of law requiring it. The need for such belief, the subjective element, is implicit in the very notion of opinio juris sive necessitatis”.
Opinio juris is reflected in acts of states (Nicaragua Case) or in omissions (Lotus Case) in so far as those acts or omissions are done following a belief that the said State is obligated by law to act or refrain from acting in a particular way. In the Lotus Case, France alleged that jurisdictional questions on collision cases are rarely heard in criminal cases because States tend to prosecute only before the flag state. France argued that this absence of prosecutions points to a positive rule in customary law on collisions.
The Court held that this “would merely show that States had often, in practice, abstained from instituting criminal proceedings, and not that they recognized themselves as being obliged to do so; for only if such abstention were based on their being conscious of having a duty to abstain would it be possible to speak of an international custom. The alleged fact does not allow one to infer that States have been conscious of having such a duty; on the other hand, as will presently be seen, there are other circumstances calculated to show that the contrary is true”.
The World Court has yet to commit itself on the key question of the nature of customary law. And it has been Delphic on the more specific question of opinio juris. In the Lotus Case, it spoke in psychological terms, holding that states must be “conscious of having a duty” in order for a rule of customary law to be present. Similarly, in the North Sea Continental Shelf Cases, the Court referred to opinio juris as “a subjective element” in the composition of customary law. More specifically, it is described as “a belief”, which appears to have a psychological flavour.
Further in this apparently psychological vein, the Court held that states “must feel that they are conforming to what amounts to a legal obligation”. The Court does not explicitly say, however, whether this subjective element or belief refers to the positions of each state individually, on its own, or to the subjective stance of the community as a whole. In later cases, the Court has held back from this overtly psychological phraseology.