7. What is opinio juris? How can it be proven?

http://www.law.cornell.edu/wex/opinio_juris_international_law

It is the second element (along with state practice) necessary to establish a legally binding custom. Opinio juris denotes (обозначает) a subjective obligation, a sense on behalf of a state that it is bound to the law in question.

ICJ Statute, Article 38(1)(b) (the custom to be applied must be "accepted as law").

Whether the practice of a state is due to a belief that it is legally obliged to do a particular act is difficult to prove objectively. Therefore, opinio juris is an unsettled and debated notion in international law.

The term is derived from the Latin phrase opinio juris sive necessitatis ("an opinion of law or necessity").

See North Sea Continental Shelf Cases (West Germany v. Neth./Denmark) (1968-69) (holding that states' frequent or habitual performance of certain actions does not, by itself, establish opinio juris).

Wikipedia

Because opinio juris refers to the psychological state of the state actor—asking why the state behaved as it did—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.

In the Paquete Habana case (decided by the United States Supreme Court in 1900 on the question of whether small coastal fishing boats are immune from capture during war time under customary international law), evidence of opinio juris included medieval English royal ordinances, agreements between European nations, orders issued to the U.S. Navy in earlier conflicts, and the opinions of legal treatise writers.

Finally, the context, circumstances, and manner in which the state practice is carried out can also be used to infer the existence of opinio juris.

As the ICJ stated in the North Sea Continental Shelf cases of 1969, “Not only must the acts concerned amount to a settled practice, but they must also be such, or be carried out in such a way, as to be evidence of a belief that this practice is rendered obligatory by the existence of a rule of law requiring it.”

Nonetheless, state's motives can change over time, and that it is not necessary that opinio juris be a significant impetus (импульс, толчок) for each instance of action.

int. committee of the red cross CUSTOMARY INTERNATIONAL HUMANITARIAN LAW VOLUME I RULES, Jean-Marie Henckaerts and Louise Doswald-Beck

Opinio juris

The second requirement for the existence of a rule of customary international law, opinio juris, relates to the need for the practice to be carried out as of right.

The particular form in which the practice and this legal conviction needs to be expressed may well differ depending on whether the rule involved contains a prohibition, an obligation or merely a right to behave in a certain manner.

When there is sufficiently dense practice, an opinio juris is generally contained

within that practice and, as a result, it is not usually necessary to demonstrate

separately the existence of an opinio juris.

Opinio juris plays an important role, however, in certain situations where the practice is ambiguous, in order to decide whether or not that practice counts towards the formation of custom. This is often the case with omissions, when States omit to act or react but it is not clear why.

An example of such a situation was analysed by the International Court of Justice in the North Sea Continental Shelf cases in which Denmark and the Netherlands argued that a customary rule existed requiring a continental shelf to be delimited on the basis of the equidistance principle, inter alia , because a number of States had done so. The Court considered that the basis of the action of those States remained speculative and that no inference could be drawn that they believed themselves to be applying a rule of customary international law.44 In other words, the States that had delimited their continental shelf on the basis of the equidistance principle had behaved in accordance with that principle but nothing showed that they considered themselves bound by it.

It is basically in such cases, where practice is ambiguous, that both the International Court of Justice and its predecessor, the Permanent Court of International Justice, have looked in particular at whether they could separately establish the existence of an opinio juris that would indicate that the ambiguous practice in fact counted towards the establishment of customary international law.

It appears that international courts and tribunals on occasion conclude that a rule of customary international law exists when that rule is a desirable one for international peace and security or for the protection of the human person, provided that there is no important contrary opinio juris .46 Examples of such conclusions are the finding by the International Military Tribunal at Nuremberg that the Hague Conventions of 1907 had hardened into customary law,47 and the finding by the ICJ in the Nicaragua case that the rule of non-intervention in the internal and external affairs of other States was part of customary international law.48 However, when there was

clear evidence of contrary opinio juris by a number of States, including specially affected ones, international case-law has held that the existence of a rule of customary international was not proven, for example, the advisory opinion of the ICJ in the Nuclear Weapons case on the issue of whether the use of nuclear weapons was illegal,49 and the ruling of the sole arbitrator in the Texaco v. Libya case on the issue of a possible change in the law relating to compensation for expropriation.50