Sources of PIL

  1. Sources of International law refer to those bodies of law that count as international law.
  2. The formally recognised sources of international law are reflected in article 38 of the Statute of the International Court of Justice.

Article 38

  1. The Court, whose function is to decide in accordance with international law such disputes as are submitted to it, shall apply:
  2. international conventions, whether general or particular, establishing rules expressly recognized by the contesting states;
  3. international custom, as evidence of a general practice accepted as law;
  4. the general principles of law recognized by civilized nations;
  5. subject to the provisions of Article 59[1], 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.
  6. This provision shall not prejudice the power of the Court to decide a case ex aequo et bono, if the parties agree thereto.


  1. Sources of law are sometimes differentiated into formal source and material source. Formal sources of law refer to rules that have general application and are legally binding on their addressees. Material sources refer to those rules which when evidenced prove the existence of rules that have general application.
  2. Question regarding the hierarchy of sources. Even though, sources (a) and (b) are more specific article 38 do not seem to provide a hierarchy. For hierarchical definition of sources of international (criminal) law see, Rome Statue of the International Criminal Court, article 21.[2]


  1. International Custom“international custom, as … a general practice accepted as law.”Thus, there are two elements of international custom. There must be a general practice which is accepted as international law. Mere usage, practices carried out of courtesy cannot be international custom.
  • Material sources of custom: texts of international instruments, decisions of international courts, decisions of national courts, national legislation, diplomatic correspondence, opinions of national legal advisors, practice of international organizations, a pattern of treaties in the same form, resolutions relating to legal questions in the U.N. GA etc.
  1. Nature of State Practices: a. Duration and Consistency: According to Crawford, it is very much a matter of appreciation. Complete uniformity of practice is not required, but substantial uniformity in practice is essential. See, Anglo-Norwegian Fisheries Case (ICJ Reports 1951) where court refused to accept the existence of a 10-mile rule for the closing line of bays.

In Nicaragua Case, the ICJ held that it was not necessary that a state practice be absolutely rigorously conform with customary rule:

In order to deduce the existence of customary rules, the Court deems it sufficient that the conduct of states should, in general, be consistent with such rules, and that instances of state conduct inconsistent with a given rule should generally have been treated as breaches of that rule, not as indications of the recognition of a new rule.

Once consistency is established, state practices need not be practiced for a long duration. A long practice is not essential. State practice in regard to space exploration, maritime delimitation are examples.

See, North Sea Continental Shelf Cases (ICJ Reports 1969, 3 at 43)

Although the passage of only a short period of time is not necessarily, or of itself, a bar to the formation of a new rule of customary international law on the basis of what was originally a purely conventional rule, an indispensable requirement would be that within the period in question, short though it may be, State practice, including that of States whose interests are especially affected, it should have been both extensive and virtually uniform in the sense of the provision invoked…


b. Generality of Practice: Complete consistency is not necessary. Lotus Case and the case of acquiescence.


5. Opinio juris: Accepted as Law

Two schools of idea:

  1. opinion juris inferred from state practices. That is, from scholarly consensus, courts and other tribunals’ previous determination. Crawford North Sea Continental Shelf Case, Nicaragua Case,
  2. Additional proof of subjective element is essential. See, Lotus Case,


Nicaragua: “In considering the instances of the conduct above described, the Court has to emphasize that, as was observed in the North Sea Continental Shelf cases, 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 the 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 this practice is rendered obligatory by the existence of a rule of law requiring it. The need for such a belief, i.e., the existence of a subjective element, is implicit in the very notion of the opinio juris sive necessitatis.”

 6. Persistent Objector: Existence of the concept enforces the state’s consent concept in international law. Evidence of objection should be clear. Existence of Rebuttable presumption.

This may take various form, objection to the whole practice or rule, or scope or limitation of rule.

7. Subsequent Objector: Rule not clear.

8. Local Customs: Right of Passage Case[3]:

It is difficult to see why the number of states between which a local custom may be established on the basis of long practice must necessarily be larger than two. The court sees no reason why long continued practice between two states accepted by them as regulating their relations should not form the basis of mutual rights and obligations between two states.


  1. Treaties
  2. Treaties and international conventions are the most important source of international law.
  3. International treaties are sometimes differentiated into two categories, “law making treaties” and “other treaties”.
  • Law making treaties create general obligations that does not end with fulfilment of the treaty obligation.
  1. Examples: Four Geneva Conventions, Genocide Convention, UNCLOS etc.


  1. Relations of Treaties to Custom
  2. Treaty provision/s may crystallize into custom.
  3. North Sea Continental Shelf Case and Baxtor Paradox.


According to Baxter, after North Sea Continental Shelf it became clear that ‘the treaty-making process may also have unwelcome side-effects’: this is the so-called Baxter paradox. In particular, he notes that treaties declaratory or constitutive of custom may ‘arrest’ its further development and that until ‘the treaty is revised or amended, the customary international law will remain the image of the treaty as it was before it was revised.’


  • Note that in North Sea Continental Shelf Case, the Court refused to find provisions that could have been unilaterally reserved by contracting states as amounting to customary rule.


“The faculty of making reservations … on the basis of the Convention: for so long as this faculty continues to exist, and is not the subject of any revision … it is the Convention itself which would, for the reasons already indicated, seem to deny to the provisions … the same norm-creating character.” (para. 72)


  1. General Principles of Law
  2. Oppenheim: “the intention is to authorise the court to apply the general principles of municipal jurisprudence, in particular of private law, insofar as they are applicable to relations of states.”
  3. According to Crawford, “Tribunals have not adopted a mechanical system of borrowing from domestic law. Rather they have employed or adapted modes of general legal reasoning as well as comparative law analogies in order to make a coherent body of rules for application by international judicial process. The result is a body of international law the content of which has been influenced by domestic law but which is still its own creation.”
  • Examples: Chorzow Factory Case, PCIJ: “One party cannot avail himself of the fact that the other has not fulfilled some obligation or has not had recourse to some means of redress, if the former party has, by some illegal act, prevented the latter from fulfilling the obligation in question, or from having recourse to the tribunal which would have been open, to him.”

Principle of Estoppel and Acquiescence, in Legality of Use of Force (Serbia and Montenegro v. Canada, (PO), ICJ Reports 2004, 444-7; Gulf of Maine Case, ICJ Reports 1984 p 246, Barcelona Traction Case (PO), ICJ Reports 1984 246, 308-9.

No one can be judge in him own suit (Interpretation of Article 3, Paragraph 2 of the Treaty of Lausanne (1925) PCIJ.

Res judicata, Effects of Awards of Compensation Made by the United Nations Administrative Tribunal, ICJ Reports 1954, p 47, 53.

  1. General Principles of International law may also indicate such principles of international law such as sovereign equality, reciprocity, finality of awards, good faith, jurisdiction etc.


  1. Judicial Decisions
  2. Not a strict source of law but evidence of the law.
  3. Important persuasive value.
  • Evidence of state practice.
  1. In Polish Upper Silesia, the PCIJ said that, “the object of [article 59] is simply to prevent legal principles accepted by the court in a particular case from being binding on other states or in other disputes.”
  2. Courts have referred to the concept of precedent in Reparation of Injuries Case, Exchange of Greek and Turkish Populations Case but have used decisions as evidence of law.
  3. Decisions of National Courts may also provide assistance to international courts in settling legal question and provide evidence of law.


  1. Other Material Sources

. The Conclusions of International Conferences

. Resolutions of General Assembly of the United Nations

. The Writings of Publicists

. ILC’s works and Codification

[1] Article 59 provides that decisions “have no binding force except between the parties and in respect of that particular case.’

[2] The Court shall apply:

  • In the first place, this Statute, Elements of Crimes and its Rules of Procedure and Evidence;
  • In the second place, where appropriate, applicable treaties and the principles and rules of international law, including the established principles of the international law of armed conflict;
  • Failing that, general principles of law derived by the Court from national laws of legal systems of the world including, as appropriate, the national laws of States that would normally exercise jurisdiction over the crime, provided that those principles are not inconsistent with this Statute and with international law and internationally recognized norms and standards.
  1. The Court may apply principles and rules of law as interpreted in its previous decisions.

[3] ICJ Reports 1960 p 6.