Sources of international law 

Unlike a domestic legal system, international law has no single legislature, no single court with universal compulsory jurisdiction, and no single enforcement agency. Before asking what international law says about a problem, a student must first ask a threshold question: where does international law actually come from? This article introduces the four recognised sources of international law and explains how they relate to one another. It is the orienting article for this module; later articles build on it to examine how international law interacts with Australia's domestic legal system.

Treaties

A treaty is a written agreement between states (or between states and certain other international actors) intended to create binding obligations under international law. Treaties may be bilateral, between two states, or multilateral, among many. A state that has become a party to a treaty is bound by it as a matter of international law, on the basis that agreements are to be honoured. Treaties are the closest international law comes to "legislation," but they bind only the states that have consented to be parties, and — as later articles in this module explain — a treaty binding a state internationally does not automatically become part of that state's domestic law.

Customary international law

Customary international law (CIL) arises not from agreement but from practice. A rule of custom is established where two elements are both present:

• state practice — a general, consistent and sufficiently widespread pattern of conduct among states; and
• opinio juris — evidence that states engage in that practice because they believe themselves legally obliged to do so, not merely out of courtesy, convenience or coincidence.

The second element is what separates a binding customary rule from a mere habit or common courtesy. A practice followed consistently but only out of convenience — for example, a diplomatic courtesy nobody believes is legally required — does not crystallise into custom no matter how widespread it becomes. Because customary international law depends on practice actually observed, it can change over time as state conduct and belief change, and it can bind even states that never explicitly consented to it, subject to limited exceptions for a state that has persistently and openly objected to an emerging rule while it was forming.

General principles of law

Where neither treaty nor custom answers a question, international tribunals may draw on general principles of law recognised by legal systems generally — concepts such as good faith, estoppel, or the idea that no one should profit from their own wrong. This source functions as a gap-filler, supplying a rule where the more specific sources are silent, by reasoning from what legal systems around the world tend to share in common.

Subsidiary means: judicial decisions and scholarly writing

Decisions of international courts and tribunals, and the writings of highly qualified international law scholars, are not sources of international law in the same sense as the three above. They do not themselves create obligations. Instead they are subsidiary means for identifying what the law already is — evidence of the content of a treaty, custom or general principle, rather than an independent origin of new legal rules. A judicial decision may be highly persuasive, and a widely cited scholarly statement of the law may shape how states and tribunals understand a rule, but both remain secondary to the primary sources they help interpret.

How the sources relate

There is no rigid formal hierarchy among treaty, custom and general principles — a treaty between two states cannot simply override a rule that has attained the status of a peremptory norm binding on all states, for example. In practice, however, arguments are usually built in a consistent order: first ask whether a treaty applies and binds the relevant states, then ask whether a rule of custom applies, and only then turn to general principles, using judicial decisions and scholarly writing throughout as evidence of what each source actually requires.

Applying this in a problem question

1. Ask first whether a treaty exists, whether the relevant states are parties to it, and what it actually says.
2. If no treaty answers the question, or a party is not bound by it, ask whether a rule of customary international law applies — and test it against both state practice and opinio juris, not practice alone.
3. If neither treaty nor custom resolves the issue, consider whether a general principle of law common to legal systems supplies an answer.
4. Use judicial decisions and scholarly writing to support your identification of the content of a source — not as a source in themselves.
5. Keep the question of what international law requires separate from the question of what, if anything, Australian domestic law requires — that second question is addressed in the companion article on treaty incorporation.