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5 Sources of Public International law [2024]

This post covers everything you need to know about sources of public international law

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Sources of Public International law

Generally, the sources of public international law are provided under Article 38(1) of the Statute of the International Court of Justice (SICJ). That article recognizes International conventions, International customs, The general principles of law, judicial decisions, and the teachings of the most highly qualified publicists as sources of Public international law.

Specifically, the said Article 38(1) states that:

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The court, whose function is to decide in accordance with International Law, such disputes as are submitted to it, shall apply:

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

Let’s now discuss one source after another

International conventions

International conventions are the first source of public international law recognized by SICJ. The term conventions also mean treaties.

Treaty is defined by article 2 (1)(a) of the Vienna Convention on the Law of Treaties (1969) to mean

An 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״

As a source of law, the treaty is founded on certain accepted principles which may be summarized as follows:

The treaty arises from the express consent of a state.

Being based on consent, the general principle is that only the parties to a treaty are bound by its terms.

However, there is a category called dispositive treaties that create an objective legal regime binding upon third state.

The treaty governs the relations of parties inter se.  This means a state signatory to the treaty is bound by the treaty in its dealings with other signatories.  It is not bound in its dealings with non-signatory states.

However, this position changes if the treaty in question will have acquired the status of customary International Law.

Where a treaty codifies customary International Law, the parties (signatories) will be bound according to the law of treaties, and non-parties (non–signatories) may be bound because of the obligations arising under customary International Law.

All treaties contain obligations for states which are parties to those treaties.  However, a distinction is always drawn between law-making treaties and treaty contracts.

Law-making treaties will purport to lay down general rules and will be multilateral in character.

Treaty contracts are treaties that resemble contracts. An example is when state ‘A’ makes a trade treaty with state ‘B’.

Read also: Public International law vs Municipal law


Custom is the second source of public international law recognized by SICJ. In international law, customs is also referred to as International customary law.

So far, jurists have demonstrated a difference between custom and usage, Usage represents a general practice that does not denote or reflect any legal obligation.  A custom may be said to arise when the practice is coupled with a legal obligation.

Sources of Custom

Customary International Law is greatly derived from two sources, namely state practice and opinio Juris.

State practice

The following are elements of state practice:

  1. Diplomatic relations between states
  2. Official statements by foreign ministers; the opinion of legal advisors; and bilateral treaties; all constitute evidence of state practice.  Similarly, statements by ambassadors or diplomatic representatives fall within this category.
  3. The practice of international organizations
  4. Since much activity between states is conducted through the auspices of international organizations, the cumulating conduct of an international organization constitutes evidence of state practice.
  5. State laws and decisions of municipal courts

In seeking evidence of state practice, one of the methods to use is to examine the judgments of municipal courts and the enactment of individual legislatures.

Opinio Juris sive necessitatis

The purpose of the doctrine of opinio juris is to demonstrate that, the right or obligation comprising the rule in question is not a mere matter of usage or practice, but recognition by states as obligatory.

Opinio Juris is actually what turns a mere usage into custom because states will behave a certain way not because they are just used to, but because they are convinced that it is binding upon them to do so.

The Test of Custom

The following are the test which is applied to prove whether a certain practice qualify to be a custom under Public international law


If the consistency and generality of practice is proven, no particular duration is required.  The passage of time will, of course, be evidence for consistency and generality which are also other tests for custom.  Therefore, it is not very necessary that a practice belongs or even immemorial

Uniformity and Consistency of the practice

For a customary rule to be recognized as a rule of International Law, it should be uniform.

In the Asylum Case [International Court of Justice – ICJ Reports (1950), P 276 -277] the ICJ pronounced that, the party which relies on a custom, must prove that this custom is established in such a manner that it had become binding on the other party, and that, it is in accordance with an instant and uniform usage practiced by states in question.

Generality of practice

One of the proofs of custom is the generality of practice among states.  The generality of practice has to do with the level of acceptance of the concept.

Therefore, customary law is thus established by virtue of A pattern of claim, absence of protest by states particularly interested in the matter at hand, and Acquiescence by other states

The General Principles of Law

Article 38(1)(c) of the statute of the International Court of Justice refers to the general principles of law recognized by civilized nations as one of the sources of International Law.

At first, the expression raised hot debate as to what is the exact meaning of civilized states. It is arguable that, in the beginning, this expression denoted only independent countries, and was excluding the bulky of dependent African countries.

Leaving that debate aside, in any system of law, a situation may well arise where a court is considering a case before it, and realizes that there is no law covering exactly that point. 

In such instances, the judge or judges will proceed to deduce a rule that will be relevant, by an analogy from already existing rules, or directly from the general principles that guide that particular legal system.

Sources or instances of general principles of law

The following are some of the sources, or instances of general principles

Recourse to Equity

Equity is one of the general principles of law found in many legal systems. Equity could be understood as a set of principles constituting the values of the legal system.

In the case of Rann of Kutch Arbitration between India and Pakistan [(1968), 50 ILR, P. 2], the arbitration agreed that equity formed part of International Law and that, accordingly the parties could rely on such principles in the presentations of their cases.

The International Court of Justice in the North Sea Continental Shelf cases [ICJ Reports, (1969) p. 3] took the same stand.

Note that, this case was decided just a year after the Indian-Pakistan Case.

General Principles of Law in the Practice of Tribunals
Arbitral Tribunals

Arbitral tribunals have frequently resorted to municipal analogies.  This could include issues of responsibility of a state for the acts of its agents, or in the assessment of damages, or any other subject matter.

The International Court of Justice

The Court has used this source (general principles) as part of its judicial reasoning.  For example, has been used most frequently and successfully the domestic law analogies in the field of evidence; procedure; and jurisdictional questions.

General Principles of International Law

This may refer to logical propositions resulting from judicial reasoning on the basis of existing International Law, as well as on the basis of municipal analogies.

There are already agreed principles of International Law such as pacta sunt servanda (bindingness of treaties); principles of consent; reciprocity; equality of states; finality of awards and settlements; the legal validity of agreements; good faith; domestic jurisdiction, etc.

Judicial Decisions and The Writings of Publicists

Article 38(1) (d) provides that, the Court shall apply judicial decisions and the teachings of the most highly qualified publicists of the various nations, as a subsidiary means for the determination of the rules of law.

Decisions of international tribunals

Judicial decisions of international tribunals are not strictly speaking formal sources, but in some instances, they are regarded as authoritative evidence of the state of the law.

Arbitral tribunals awards

In numerous awards given by arbitral tribunals, there are notable contributions to the development of the law by eminent jurists sitting as arbitrators, umpires, or commissioners.

Reference to arbitral awards by the international court of justice

The Court has referred to the jurisprudence of international arbitrations

Decisions of the International Court of Justice

The general trend is that, the ICJ applies the law and does not make it (no precedent).

However, a unanimous decision made by the Court has a role in the progressive development of the law. An example is the decisive influences of decisions of the Court in the following cases in the development of International Law:

  1. The Reparations Case
  2. The  Genocide Case
  3. The Fisheries Case, and
  4. The Nottebohm Case

On a few occasions, the Court has referred to particular decisions of itself.

Decisions of Nations Courts

Article 38 (1)(d) of the statute of the ICJ is not confined to international decisions only.  Decisions of national courts and tribunals have value.  For example, some decisions provide direct evidence of state practice; state succession; sovereign immunity; diplomatic immunity; extradition, war crimes, etc…

The Writings of Publicists

In principle, the writings of publicists only constitute evidence of the law.  However, in some areas individual writers have had a formative influence, e.g. Gidel Gilbert has had some formative influence on the law of the sea.

Not forgetting the need for caution, the opinions of publicists are used widely.


O’BRIEN JOHN, International Law, London Cavendish Publishing Limited, 2001.

KU CHARLOTTE & DIEHL PAUL, (Editors), International Law: Classic and Contemporary Readings, 2nd Edition, London, Lynne Rienner Publishers, 2003.

SHAW  MALCOLM,   International Law, 5th Edition, Cambridge University Press, 2005.

Isack Kimaro
Isack Kimaro

Isack Kimaro, a lawyer, Creative Writer and self-taught SEO expert has been a prominent author of law-related topics since 2017. Through hard work, dedication, and a relentless pursuit of knowledge, Isack has successfully navigated the legal industry by providing valuable and easy-to-understand legal information to 500,000+ individuals of all levels of understanding.