Lesson 3: Principles of International Law: Sources of International Law

THE PRINCIPLES OF INTERNAL LAW.

Dated: 05/03/2026

Topic: Sources of international law

The sources of international law are found in Article 38(1) of the Statute of the International Court of Justice (ICJ Statute).

Article 38(1) ICJ Statute

Article 38(1) lists the authoritative sources that the ICJ applies when deciding disputes between states.

It provides that the Court shall apply:

• International Conventions (Treaties)

These are agreements between states that create legally binding obligations.

Example: the United Nations Charter.

International Custom (Customary International Law)

General practice of states accepted as law (opinion juris).

Example: the customary rule prohibiting the use of force against another state's territorial integrity.

General Principles of Law Recognized by Civilized Nations

Fundamental legal principles common to national legal systems.

Examples include good faith, equity, and res judicata.

Judicial Decisions and the Teachings of the Most Highly Qualified Publicists (Subsidiary means)

These help interpret and determine the law but are not primary sources.

Example: judgments from the International Court of Justice such as the Military and Paramilitary Activities in and against Nicaragua (Nicaragua v. United States) case.

Structure of the Sources Scholars generally classify them as:

• Primary Sources
• Treaties & Convention
• Customary international law
• General principles of law
• Subsidiary Sources
• Judicial decisions
• Writings of jurists (publicists)

Importance in International Law Article 38 is often described as the “constitutional provision” of international law, because it identifies the legal materials courts rely on when determining international legal rules.

Is the UDHR Binding?

The Universal Declaration of Human Rights (UDHR) was adopted in 1948 by the United Nations General Assembly.

Technically, the UDHR is not a treaty, so it is not legally binding by itself.

It is a declaration, meaning it was originally intended as a moral and political guideline for states.

However, over time many of its provisions have become binding because they have developed into customary international law.

Examples of UDHR rights widely recognized as customary law include:

Right to life Prohibition of torture Prohibition of slavery Equality before the law These norms are now widely accepted by states and international courts.

Customary International Law* Customary international law is a rule derived from consistent state practice accepted as law.

This source of law is recognized under Article 38(1)(b) of the Statute of the International Court of Justice.

It binds all states, even if they have not signed a treaty, provided they are not persistent objectors.

Two Characteristics (Elements) of Customary International Law

For a rule to become customary international law, two elements must exist:

• 1. State Practice

This means consistent and general practice by states.

Examples include:

• National legislation Diplomatic actions Military conduct Judicial decisions Government statements The practice must be widespread and reasonably consistent.

Example:

The prohibition on supporting armed rebels in another state was discussed in the Military and Paramilitary Activities in and against Nicaragua (Nicaragua v. United States) case.

The Court confirmed that consistent state behavior can create customary law.

2. Opinion Juris (Acceptance as Law)

This means states follow the practice because they believe they are legally obliged to do so, not just out of convenience or habit.

In simple terms:

State practice = what states do Opinion juris = why states do it Both elements must exist together for a rule to become customary international law.

General Principles of Law

Another source under Article 38(1)(c) is general principles of law recognized by civilized nations.

These are legal principles common to most national legal systems.

Examples include:

• Good faith Equity Res judicata (a matter already judged)
• Responsibility for wrongdoing These principles help fill gaps in international law when treaties or customs are unclear.