Sources of International Law

Art. 38 (1) of the International Court of Justice (ICJ) Statute provides a list of sources of International Law. However, the list is not complete and the statue doesn’t establish any hierarchy or sources: it is made by an evaluation of mere convenience, “although international courts and tribunals will, as a matter of convenience, invoke international treaties first”. 

Moreover, the relationship between sources is made case-by-case, and it is based on the principles of interpretation. For example, if two laws cover the same situation, the law covering the specific subject matter, could override a law that covers a general matter (lex specialis derogat legi generali rule). Furthermore, it should be underlined that there is the view that conflict of norms should be avoided by systematic interpretation, which means we should think about these laws as a whole. 

The sources indicated by the ICJ are the following:

  1. Treaties: 

Whether bilateral or multilateral, general or particular, “establishing rules recognized by the contesting States Parties to a dispute before the ICJ”

The Vienna Convention on the Law of Treaties (1969) is very flexible about the terms used to indicate this type of agreement (i.e. Convention, Agreement) and they are all used interchangeably. However, the Convention provide a definition of “Treaty”. Indeed, the Vienna Convention (VC) said in section 1(a) that ” treaty” means “international agreement concluded between States in written form and governed by international law, whether embodied in a single instrument or in two or more related instruments and whatever its particular designation”.

What it is important about treaties is that Member States decide to commence the process and they decide on the scope of the treaty. Moreover, they are the ones who will adopt them and implement them, having a large consent among Member States. 

There is a hierarchy of which entity can make a statement: Member States (original group) and then Member States, then Intergovernmental organization, and NGOs making statements. 

Treaties are more exclusive and specific than customary law, but between them there is a vivid interaction, as well as an interrelationship between treaties. This is important and it is explained by the fact that a treaty is a “Living Document”. According to VC 31 (3)(B) it can be “amended, interpreted and changed via practice”.

  1.  Customary international law, divided in: general and special/local, “as evidence of a general practice accepted as law”.

There are two elements that apply case-by-case, one objective (State practice) and one subjective (Opinio Juris). 

The first element consists in the constant repetition over time of a given behavior by the generality of the subjects (diuturnitas); the second element is related to the belief that that behavior conforms to law or necessity.

The second element, opinio juris, is comprehensive of government legal opinions, diplomatic correspondence, resolutions adopted by International organization and intergovernmental conferences. The UNGA resolutions, For example, are nor binding at all. can be considered as the result of international will.  That’s why the UNGA Resolutions are taken so seriously, and it would be wrong thinking they are part of the soft law.  

In compare to the Treaties, “the development and adjustment of customary international law is more flexible than the development of treaty law; and second, customary international law is, by its very nature, universal, whereas treaty law binds the parties to these treaties only (D’Amato 12)”

Moreover, unlike treaties, which are valid only in relations between the parties, customary norms oblige all international subjects, regardless of participation in the formative process. “Non-governmental organizations have no impact on the formulation of customary international law as long as their actions are not directly attributable to States. However, to the extent that such actors are engaged in the work of international organizations, they can at least indirectly influence the formulation of customary international law”.

  1. General principles of law recognized by civilized nations

“International and regional courts and tribunals make use of principles as an interpretative tool or as a source of concrete obligations”. Indeed, “they guide the interpretation of international treaties and, due to their abstract formulation, are the gateway for progressive interpretation”.

General principles of law are used primarily as “gap fillers” when treaties or customary international law do not provide a rule of decision. Examples of these general principles of law are laches, good faith, res judicata, and the impartiality of judges. International tribunals rely on these principles when they cannot find authority in other sources of international law.

 “Principles may be derived from municipal law, from general considerations, or, by generalizing, from a particular treaty regime. The development and recognition of such principles does not depend on the will of States and all States equally contributing to their development”.

The list of sources in Article 38 of the Statute is frequently criticised for being incomplete. However, we have example of other sources:

Judicial decisions: article 38(1)(d) refers to judicial decisions as a subsidiary means for the determination of rules of law. In contrast to the position in common law countries, there is no doctrine of binding precedent in international law. Indeed, the Statute of the ICJ expressly provides that a decision of the Court is not binding on anyone except the partiers to the case in which that decision is given and even then only in respect of that particular case (Article 59). Nevertheless, the ICJ refers frequently to its own past decisions and most international tribunals make use of past cases as a guide to the content of international law, so it would be a mistake to assume that “subsidiary” indicated a lack of importance. Article 38(1)(d) does not distinguish between decisions of international and national courts. The former are generally considered the more authoritative evidence of international law on most topics (though not those which are more commonly handled by national courts, such as the law on sovereign immunity). But decisions of a State’s courts are a part of the practice of that State and can therefore contribute directly to the formation of customary international law.UNGA Resolutions: the United Nations General Assembly has no power to legislate for the international community; its resolutions are not legally binding. However, many of those resolutions have an important effect on the law-making process. Some resolutions are part of the treatymaking process, attaching a treaty text negotiated in the framework of the United Nations and recommended to the Member States by the Assembly (this was the case with the Convention against Torture). While it is the treaty which creates the legal obligation – and then only for the States which choose to become party to it – the importance of the United Nations in the process of creating that treaty should not be underestimated.

1. Rüdiger Wolfrum, ‘Sources of International Law’, in Max Planck Encyclopedia of Public International Law (May 2011), Paragraph 11.
2. Ibrid, Paragraph 9.
4. Id.
5. Rüdiger Wolfrum, ‘Sources of International Law’, in Max Planck Encyclopedia of Public International Law (May 2011), Paragraph 9.
6. Ibrid, Paragraph 22.
7.  Ibrid, Paragraph 28.
8. Ibrid, Paragraph 34.
9. Ibrid, Paragraph 39.
12. Rüdiger Wolfrum, ‘Sources of International Law’, in Max Planck Encyclopedia of Public International Law (May 2011), Paragraph 35
14. Ibrid

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