The sources of international law are listed in article 38.1 of the 1945 Statute of the International Court of Justice (ICJ):

Article of the Statute of the International Court of Justice, listing the sources of international law.
Statute of the ICJ, article 38

But what are exactly these sources? How are they decided or created and then implemented?

This article is meant to briefly answer such questions, in a clear manner. Don’t hesitate to leave if comment through the contact form if you have any constructive criticisms!

a. International conventions

The term “convention” has two meaning:

  • A generic meaning referring to conventions in general as sources of international law;
  • A specific meaning referring to multilateral treaties with several parties.

The first meaning is used in the Statute. This “generic” term refers to all international agreements, even if the word “treaty” or “declaration” is put in the document.

For example, the International Covenant on Civil and Political Rights is a “convention” with regard to the Statute’s classification. These sources are consequently called conventional law -as opposed to customary law that will be detailed below.

Like to word “convention”, the word “treaty” has two meanings: a specific one and a generic one. According to the generic meaning, the word treaty embraces “all instruments binding at international law concluded between international entities, regardless of their formal designation”.

There is sometimes a confusion between the words treaty and convention as used in their generic meaning.

b. International custom

Customary law arises from international practice -or “custom”. For this practice to become law, two things are essential:

  • It must be an established practice, meaning it has been applied many times and by many States (this is the material element);
  • The practice must be recognized as creating an obligation -the States respect it because there is a sense of legal obligation (this is the opinio juris).

Once customary law is identified, it is often -but not always- codified (put in writing). For example, the practices relating to the sea have been codified in the 1982 United Nations Convention on the law of the sea -known as the Montego Bay Convention.

c. General principles of law recognized by civilized nations

These are principle that existing at the domestic level – or national/State level. These principles are adapted to the international legal order.

In 2018, the International Law Commission decided to include the general principles of law in its program of work, leading to a report from which I extracted a few key elements:

  • There are two kinds of general principles of law: those derived from national legal systems and those formed within the international legal system;
  • The principles are often viewed as “a supplementary source of international law in the sense that they serve to fill gaps in conventional and customary international law, or to avoid findings of a non liquet.” (non-liquet means something being unclear).

A few principles quoted in the report are:

  • principle of fair trial;
  • general principle of the direct applicability of international law with respect to individual responsibility and punishment for crimes under international law;
  • general principle of the autonomy of international law over national law with respect to the criminal characterization of conduct constituting crimes under international law.

d. Judicial decisions and teachings of the most highly qualified publicists of the various nations

When listing out the sources of international law, this one is often left behind. This is why you will often hear/read about the “three sources of IL” instead of “four”.

This one is a bit tricky as the text of the ICJ Statute itself mentions this is “subject to provisions of article 59”. So, what is written in article 59?

Article of the Statute of the International Court of Justice, on the binding forces of the court's decisions
Statute of the ICJ, article 59

Not really helpful, is it? It means that decisions of the Court are not “international law” because the said decision is only applicable to a specific context and case. But it does not give any information on other “judicial decisions” nor on the teachings.

Here are a few notes:

  • judicial decisions are international or national court’s judgments (also referred to as case-law, legal precedent or jurisprudence). However, as shown in article 59 of the Statute, courts’ decisions often do not have a general scope as they only apply to a specific case.
  • teachings are the thoughts of scholars on legal issues. The Statute only refers to the ones of the “most highly qualified publicists”.

Judicial decisions and teachings have a similar impact:

  • Though these, we can identify international custom;
  • Identify general principles of law; and
  • These are auxiliary means in the determination of law (meaning they are used/referred to only when no other source of law deals with the issue).

NOTE: There is no hierarchy within the sources of international law.