Sources of International Law — Article 38 of the ICJ Statute — Public International Law Notes

Sources of International Law — Article 38 of the ICJ Statute

When the World Court wants to know what the law is on any dispute, it opens a single provision: Article 38(1) of its own Statute. It is the nearest thing international law has to a table of contents. If a student remembers only one article in the whole subject, it should be this one — because “Sources” is the highest-frequency topic in Unit I, appearing on almost every sitting.

Where the rules come from — Article 38 is the answer sheet

Since there is no world legislature, the question “where do the rules come from?” has to be answered by pointing to the accepted sources. The classic list is Article 38(1) of the Statute of the International Court of Justice (1945), which directs the Court to apply, in this order:

Article 38(1), Statute of the ICJ (1945): The Court shall apply — (a) international conventions… 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) … 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.

In Simple Terms: The Court looks first at treaties (written agreements between the parties), then at custom (what States actually do out of a sense of legal duty), then at general principles common to national legal systems, and finally — only to help interpret the first three — at past judgments and the writings of leading jurists.

Now take each source.

A. International conventions (treaties) — Art. 38(1)(a)

A treaty is a written agreement between States, binding on the parties (pacta sunt servanda). Treaties are the clearest and fastest-growing source. Law-making treaties (e.g. the UN Charter, the Geneva Conventions) lay down general rules for many States; treaty-contracts settle a matter between two or a few States. A treaty binds only the parties (pacta tertiis nec nocent nec prosunt — a treaty neither harms nor benefits a third party), though a treaty rule may pass into custom and so bind non-parties.

B. International custom — Art. 38(1)(b)

Custom is the oldest source and carries huge marks, so learn its two elements:

Usage / State practice (the material element) — a general, consistent and uniform practice by States over time (how they act: diplomatic conduct, legislation, decisions, treaties). It need not be ancient (see North Sea Continental Shelf, 1969) but it must be settled.
Opinio juris sive necessitatis (the psychological element) — the belief that the practice is followed because it is legally obligatory, not from mere courtesy or convenience. This is what separates a custom (law) from a mere usage or habit (comity).

Both elements are required. A State may escape a customary rule if it was a persistent objector — it consistently objected while the rule was forming.

C. General principles of law recognized by civilized nations — Art. 38(1)(c)

These are principles found in nearly all national legal systems, which the Court can borrow to fill gaps where there is no treaty or custom — for example res judicata [a matter already decided cannot be re-litigated], good faith, estoppel, that a person cannot benefit from his own wrong, and the duty to make reparation for a wrong (Chorzów Factory, 1928).

D. Judicial decisions and juristic writings (subsidiary means) — Art. 38(1)(d)

These do not create law; they are subsidiary means to determine what the law is. “Judicial decisions” means ICJ and PCIJ judgments (there is no strict stare decisis [binding precedent] in the ICJ — Art. 59 says a decision binds only the parties — but earlier judgments carry great weight), arbitral awards, and even national court decisions as evidence of practice. “Teachings of the most highly qualified publicists” means the writings of leading jurists.

E. Article 38(2) — equity (ex aequo et bono)

Under Article 38(2) the Court may decide a case ex aequo et bono [according to what is just and good] — i.e. on fairness rather than strict law — but only if the parties agree. It has never actually been used.

Two further modern sources not listed in 1920 but now important: resolutions/decisions of international institutions (UN General Assembly resolutions can evidence opinio juris and help crystallise custom, though they are generally only recommendatory), and jus cogens (peremptory norms).

⚠️ DON’T CONFUSE — usage vs custom

A usage is a habitual practice followed from courtesy, convenience or comity — it creates no legal obligation (e.g. certain ceremonial honours). A custom is a usage that States follow because they feel legally bound (opinio juris), and it is binding law. The exam trap is to define custom as “a long practice” and stop — that describes a usage. Always add the second element: the opinio juris.

🧩 WORKED EXAMPLE — proving a rule of custom

Facts. State A claims a customary right for its warships to pass through a strait. State B denies any such custom exists.

Rule. A rule of custom needs (i) general, consistent State practice + (ii) opinio juris (belief it is legally required) — Art. 38(1)(b); North Sea Continental Shelf (1969).

Apply. A must show that States generally allow such passage and do so out of a sense of legal duty. If passage is merely tolerated as a courtesy, the second element (opinio juris) fails and there is only a usage, not a custom binding on B.

Conclusion. No custom unless both elements are proved. If B was a persistent objector while the practice formed, the rule does not bind B even if the custom otherwise exists.

flowchart TD
    A["Sources of International Law<br/>Art. 38(1) ICJ Statute"]
    A --> B["Treaties / conventions<br/>Art. 38(1)(a)"]
    A --> C["Custom<br/>Art. 38(1)(b)"]
    C --> C1["Usage / practice"]
    C --> C2["Opinio juris"]
    A --> D["General principles<br/>Art. 38(1)(c)"]
    A --> E["Judicial decisions + jurists<br/>Art. 38(1)(d) — subsidiary"]
    A --> F["Equity ex aequo et bono<br/>Art. 38(2) — only if parties agree"]
    classDef box fill:#e8f0fe,stroke:#333,color:#111;
    class A,B,C,C1,C2,D,E,F box;

Case Laws

North Sea Continental Shelf Cases (Germany v. Denmark/Netherlands, 1969) — custom needs settled practice plus opinio juris; a treaty rule can pass into custom only if it is of a fundamentally norm-creating character.
The Paquete Habana (1900) — the exemption of coastal fishing vessels from capture is established by ancient, continuous usage accepted as law; “international law is part of our law”.
Asylum Case (Colombia v. Peru, 1950) — a regional or local custom must be proved by the State asserting it; the practice must be constant and uniform and accepted as legally binding.
Chorzów Factory Case (1928, PCIJ) — the duty to make reparation for a breach is a general principle of law; “reparation must wipe out all the consequences of the illegal act”.

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