The early positive school emphasized the importance of custom and treaties as sources of international law.

On the question of preference between sources of international law, rules established by treaty will take preference if such an instrument exists.

(See also: Sources of international law)

Sources of international law are the materials and processes out of which the rules and principles regulating the international community are developed.

Article 38(1) of the Statute of the International Court of Justice lists the general sources of international law:

None of the sources of international law cited by the Plaintiffs were a proper predicate for jurisdiction under the Alien Tort Claims Act.

In the morning he could, using the most fearful language, prosecute a former Bolshevik comrade, and in the afternoon he could prepare a learned article on the sources of international law.

The scholarly works of prominent jurists are not sources of international law but are essential in developing the rules that are sourced in treaties, custom and the general principles of law.

Along with general principles of law and treaties, custom is considered by the International Court of Justice, jurists, the United Nations, and its member states to be among the primary sources of international law.

Given the limits of treaties or custom as sources of international law, Article 38(1) may be looked upon as a directive to the Court to fill any gap in the law and prevent a non liquet by reference to the general principles.

It is also argued however that international treaties and international custom are sources of international law of equal validity; this is that new custom may supersede older treaties and new treaties may override older custom.

Certainly, judicial decisions and juristic writings are regarded as auxiliary sources of international law, whereas it is unclear whether the general principles of law recognized by 'civilized nations' should be recognized as a principal or auxiliary source of international law.

A peremptory norm or jus cogens (Latin for "compelling law" or "strong law") is a principle of international law considered so fundamental that it overrides all other sources of international law, including even the Charter of the United Nations.

In addition, unlike the Article 21 of the Rome Statute of the International Criminal Court, which clearly defines hierarchy of applicable law (or sources of international law), the language of the Article 38 do not explicitly support hierarchy of sources.

In other words, the application and enforcement of the common heritage of mankind require a critical reexamination of many well-established principles and doctrines of classical international law, such as acquisition of territory, consent-based sources of international law, sovereignty, equality, resource allocation and international personality."