Sources of Public International Law can be classified into – (1) International Conventions; (2) International Customs; (3) General Principles of Law recognized by civilized nations; (4) Decisions of Judicial or Arbitral Tribunals; (5) Juristic Works and (6) Decisions or Determinations of the Organs of International Institutions.
The first five sources are mentioned in Article 38 of the Statute of International Court of Justice and the sixth source is conspicuous from the said Article as it was not sufficiently developed when this Article was drafted and enacted for the first time.
The term “Convention” applies to any treaty, protocol or agreement, regardless of its title or form. According to Article 38 of the Statute of International Court of Justice, it is the first source of International Law. They are the most important source of public international law.
International treaties may be classified into –
- Law making treaties – These are those treaties which are entered into by a large number of States. These are the direct sources of international law. Law- making treaties are divide into –
- Treaties enunciating the rules of Universal International Law – those treaties which are signed by a majority of the States are called treaties enunciating the rules of universal international law. United Nations Charter is an example of such treaties.
- Those enunciating general Principles – treaties which are entered into by a large number of countries enunciated general principles of international law 1958 and 1960. Geneva Conventions are example of such treaty.
- Treaty Contracts – Treaty Contracts are those treaties which are entered into by two or more states. The provisions of such treaties are binding only on the parties of the treaty. Such type of treaties is also source of International Law because they help in the development of customary rules of International Law.
International Customs –
International Custom used to be the most important source of International law, but their importance has lessened. Custom is a habit which has been repeated for a long time and has ultimately assumed the force of law. Usage is the earlier stage of development of custom. By usage we mean those habits that are often repeated by the States. Custom begins where usage ends.
Customary rules of international law has been developed in the following three circumstances – (a) Diplomatic relations with the States; (b) Practice of Organs of International Institutions; and (c) State Laws, decisions of the State’s Courts and State’s Parliamentary or administrative practices.
Elements of Customs –
- Evidence of a General Practice accepted as a law– Long duration is an essential element of custom in Municipal Law. But this is not necessary for an international custom. Article 38 of the International Court of Justice directs that the world court to apply international custom as a evidence of general practice accepted as a law.
- Uniformity and Consistency – the Custom should be uniform and consistent. There must be substantial uniformity and complete uniformity is not necessary.
- Generality of Practice – though universality of practice is not necessary, the practice should have been generally observed or repeated by numerous states.
- Opinio juris ncessitatis – International Court of Justice in North Sea Continental Case held that customary practice, even when it is general and consistent is not customary law unless an opinio juris is present, that is to say, unless the practice is recognized as being required by international law.
In the South West African case, Judge Tanaka observed that, in modern times the importance of custom as a source of international law has greatly reduced, with the simultaneous rise in the importance of treaties and conventions for the development of International law.
General Principles of Law recognized by the civilized States –
According to Article 38 of the Statute of International Court of Justice, the general principles of law recognized by the civilized nations as also one of the sources of international law. It is an important source as through it international law adapts itself in accordance with the changing times and circumstances.
Important case laws relating to general principles recognized by civilized States are –
R. Key, In this case the court ruled that International law is based on justice, equity and good conscience which have been accepted by long practice of States.
Barcelona Traction Case, Preliminary Objections – In this case also the International Court of Justice applied the principle of estoppels.
Chorzow Factory (Indemnity case) – In this case the Permanent Court of International Justice applied the principle of res judicata also held that one who violates a rule is liable to make reparation.
Good faith, responsibility, prescription, in the absence of any express provision to the contrary, every court has the right to determine the limits of its own jurisdiction, res judicata are some of the general principles recognized by the civilized states.
Decisions of Judicial and Arbitral Tribunals
According to Article 38 of the International Court of Justice, they are considered to be subsidiary means for the determination of rules of law. Since the doctrine of precedent does not apply in field of International law the international judicial decisions are not generally binding.
Juristic Works –
If the court does not find any treaty or judicial decision or legislative Act or any established custom, the court may take the help of jurists as subsidiary means for the determination of rules of International Law.
Decisions or determinations of the organs of International Institutions
In view of the constant change in the form and content of international law, international organizations have also become a subject of International law. After the establishment of U.N. most of the development of international law and its codification has taken place through the instrumentality of international organizations.
International Comity, State Papers, Reason, Equity and Justice are some other subsidiary sources of International Law. According to Article 38 of the Statute of International Court of Justice, the use of the sources of International Law is –
- International Conventions
- International Customs
- General Principles of Law recognized by the civilized nations
- Judicial decisions and juristic opinions
Article 38 did not establish a rigid hierarchy. In applying a provision in a convention, the Court may have to take into consideration the customary law prevailing when the convention was entered into, or general principles of law, as well as judicial precedents. It was correctly pointed in the case of Nicaragua v. U.S.A that International laws is not hierarchical but are necessarily complimentary and interrelated.
Dr. S. K. Kapoor, “International Law and Human Rights (Nutshell)” (Central Law Agency, 18th edn) 18
 I.C.J. Rep. 1969, p.3
 I.C.J. Rep. (1966) p. 6 at p. 278
  2 Ex. D. 63
 I.C.J. Rep. , p.6
 Pub. P.C.I.J., Series A, No. 17
 I.C.J  Rep. 14
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