The authoritative listing of sources is found under Article 38(1) of the Statute of the International Court of Justice (ICJ).
1. Primary Sources of International Law
- International Conventions & Treaties (Art 38(1)(a)): Written agreements between states that create explicit, binding obligations.
- Customary International Law (Art 38(1)(b)): General practice accepted as law. It requires two essential elements:
- Diuturnitas (State Practice): Constant and uniform usage across states over a period of time.
- Opinio Juris sive Necessitatis: The subjective belief by states that their practice is legally compulsory under international law.
- General Principles of Law (Art 38(1)(c)): Principles recognized in domestic legal systems (e.g., res judicata, estoppel, good faith).
📜 Landmark Case: North Sea Continental Shelf Cases (1969)
The ICJ held that for a treaty rule to transform into Customary International Law, the state practice must be constant, virtually uniform, and accompanied by a clear showing of Opinio Juris—proving that states acted out of a sense of legal obligation, rather than mere convenience or courtesy.
2. Subsidiary Sources
- Judicial Decisions & Teachings (Art 38(1)(d)): Subsidiary means for the determination of rules of law, including decisions of the ICJ (subject to Art 59 - no binding precedent) and works of highly qualified publicists.
3. Jus Cogens (Peremptory Norms)
Under Article 53 of the Vienna Convention on the Law of Treaties (VCLT), a Jus Cogens norm is a peremptory norm of general international law accepted and recognized by the international community of states as a whole as a norm from which no derogation is permitted.
- Any treaty that conflicts with a Jus Cogens norm is completely void.
- Examples: Prohibitions against aggression, piracy, genocide, slavery, and torture.