[2] It requires the Court to apply, among other things, (a) international conventions, whether general or particular, establishing rules expressly recognized by the contesting states; (b) international custom, as evidence of a general practice accepted as law; (c) the general principles of law recognized by civilized nations; (d) subject to the provisions of Article 59, judicial decisions and the teachings of the most highly qualified publicists of the various nations, as subsidiary means for the determination of rules of law.
[1] Archived 2011-06-29 at the Wayback Machine During the 19th century, it was recognized by legal positivists that a sovereign could limit its authority to act by consenting to an agreement according to the principle pacta sunt servanda.
Once it is established that some of these broad instruments are frequently shared rules found in domestic systems, they can be utilised in international law as well.
While the purpose is to establish a code of general application, its effectiveness depends upon the number of states that ratify or accede to the particular convention.
Most multi-lateral treaties fall short of achieving such a near-universal degree of formal acceptance and are dependent upon their provisions being regarded as representing customary international law and, by this indirect route, as binding upon non-parties.
Article 38(1)(b) of the ICJ Statute refers to "international custom" as a source of international law, specifically emphasizing the two requirements of state practice plus acceptance of the practice as obligatory or opinio juris sive necessitatis (usually abbreviated as opinio juris).
Derived from the consistent practice of (originally) Western states accompanied by opinio juris (the conviction of States that the consistent practice is required by a legal obligation), customary international law is differentiated from acts of comity (mutual recognition of government acts) by the presence of opinio juris (although in some instances, acts of comity have developed into customary international law, i.e. diplomatic immunity).
In its most extreme form, this would involve rejecting what states say as practice and relegating it to the status of evidence of opinio juris.
[14] There have been a number of occasions on which the ICJ has rejected claims that a customary rule existed because of a lack of consistency in the practice brought to its attention.
[20] Demand for rules that are responsive to increasingly rapid changes has led to the suggestion that there can be, in appropriate circumstances, such a concept as "instant custom".
Even within traditional doctrine, the ICJ has recognized that passage of a short period of time is not necessarily a bar to the formation of a new rule.
"[22] In cases where practice (of which evidence is given) comprises abstentions from acting, consistency of conduct might not establish the existence of a rule of customary international law.
The fact that no nuclear weapons have been used since 1945, for example, does not render their use illegal on the basis of a customary obligation because the necessary opinio juris was lacking.
[23] Although the ICJ has frequently referred to opinio juris as being an equal footing with state practice,[24] the role of the psychological element in the creation of customary law is uncertain.
The European Court of Human Rights has stressed the international public policy aspect of the jus cogens.
The significance of general principles has undoubtedly been lessened by the increased intensity of treaty and institutional relations between states.
According to Article 38(1)(d) of its Statute, the ICJ is also to apply "judicial decisions and the teachings of the most highly qualified publicists of the various nations, as subsidiary means for the determination of rules of law".
In practice, the International Court of Justice does not refer to domestic decisions although it does invoke its previous case-law.
Article 38(1)(d) of the International Court of Justice Statute states that the 'teachings of the most highly qualified publicists of the various nations' are also among the 'subsidiary means for the determination of the rules of law'.
This is accepted practice in the interpretation of international law and was utilized by the United States Supreme Court in The Paquete Habana case (175 US (1900) 677 at 700–1).
In the practice of the International Court of Justice, citations to teachings in decisions are exceptional, but Judges routinely refer to them in their individual opinions.