Sources of International Law

Author: Craig Eggett

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A. IntroductionEdit

B. The Concept of a 'Source' of International LawEdit

As a preliminary point, it is useful to consider the precise meaning of ‘source’, as some authors have used the term to describe a range of foundational aspects and processes of the international legal system. Some scholars have understood this term to cover the origins and rationale of international law as such. The use of the term ‘source’ to include the background and objectives of a rules-based international order is broader and rather unconventional. More commonly, when international lawyers consider the sources of international law, they are concerned with the processes through which international legal rules are created. The focus is on the procedures and practices that lead to the formation of international law, thereby allowing them to identify the binding law that regulates the behaviour of actors in the international legal system.

I. Sources and the Legal SystemEdit

The international legal system, like all legal systems, is composed of primary and secondary rules. Primary rules are those that create obligations, grant rights, or change a legal situation. Examples include the prohibition of the use of force, rules on human rights, and provisions that set conditions for membership to international organisations. Conversely, secondary rules are those that regulate the creation, modification, and application of other rules. Examples include rules on the interpretation of treaties and the law of state responsibility. The rules on the sources of international law are a category of secondary rules; they set out the criteria for the creation of other international rules. The presence and operation of secondary rules is indispensable for the existence and functioning of the international legal system. As such, when searching for answers to questions about the sources, it is necessary to confront difficult questions about the foundations of international law as a normative order. In turn, the theory and practice of the sources plays a role in shaping international law as a legal system. In other words, there is a co-constitutive and mutually influential relationship between the sources of international law and the international legal system. As will be demonstrated in the sections that follow, sources questions touch on issues such as the functions of different international actors, including the continued dominance of states as participants in the legal system, and the relationship between international legal norms.

II. Categories of Sources?Edit

There is a tendency to attempt to delineate between different categories of sources of international law. Most commonly, authors have distinguished between formal and material sources of law. Formal sources are the methods or process of law-making that result in binding legal rules. The formal sources of international law provide criteria against which the validity of a prospective rule is to be judged. If these criteria are fulfilled, there is a valid and legally binding rule of the system. If not, not such rule is to be found. Examples include the procedures for the formation of a treaty as reflected in the Vienna Convention on the Law of Treaties 1969 (VCLT 1969). Conversely, material sources do not in and of themselves create binding legal rules, but may provide evidence for the existence of such rules and their content. Material sources include a broad range of instruments that shed light on whether the criteria set out in the formal sources have been fulfilled. Examples include some resolutions of international organisations, the output of the International Law Commission (ILC), and judicial decisions.

Other categories that have been suggested include a division between primary and secondary (or subsidiary) sources. These terms may be used to describe the same distinction – between criteria for the creation of binding rules and the evidence for the fulfilment of such criteria – reflected in the formal/material dichotomy. It should be noted with caution that the use of the terms ‘primary’ and ‘secondary’ in this way is distinct from the description of primary and secondary rules referred to above. It is also important to note that this use of primary and secondary should not be taken to imply a formal and strict hierarchy between the sources as may be implied from such use in other legal systems. The question of hierarchy between sources and norms is dealt considered below.    

C. Article 38 of the Statute of the International Court of JusticeEdit

Article 38 of the Statute of the International Court of Justice (ICJ Statute) is the traditional, and perhaps inevitable, starting point for an examination of the sources of international law. As such, it is worth reproducing the text of this provision in full:

1.    The Court, whose function is to decide in accordance with international law such disputes as are submitted to it, shall apply:

a)    international conventions, whether general or particular, 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)    subject to the provisions of Article 59, 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.

2.    This provision shall not prejudice the power of the Court to decide a case ex aequo et bono, if the parties agree thereto.

As evidenced by the opening sentence, this provision is, strictly speaking, merely the applicable law (lex arbitri) provision of the ICJ. That being said, Article 38 has traditionally been viewed as an authoritative statement of the sources of international law. The continued accuracy of this position can be challenged and is considered both in this chapter and the ones that follow, yet it is evident that any discussion of the sources should include an examination of Article 38 ICJ Statute. This provision is, at a basic level, composed of three main parts. First, Article 38(1)(a)-(c) sets out the (formal or primary) sources of international law: treaties, customary international law, and general principles of law. Second, Article 38(1)(d) sets out the “subsidiary means” for the determination of international rules. Third, Article 38(2) allows for the Court to resolve a dispute before it on the basis of (a form of) equity, should the parties agree.

The specificities of each of these sources is considered elsewhere, so it suffices to make a handful of general observations here. On the traditional understanding of international law, it is states who play the exclusive, or at least dominant, role in the formation of international rules and an initial reading of Article 38 seems to confirm this. Indeed, the mainstream view has traditionally been that states are not bound by international rules unless they have consented to them. While it is clear that states remain prominent actors in international law-making, it can be now be legitimately questioned whether the creation of rules remains the sole prerogative of states. This question is taken up below and features in the chapters on the individual sources.

Article 38(1)(d)’s reference to “subsidiary means” reflects the aforementioned distinction between formal and material sources. That is, the means referred to in this provision – judicial decisions and the teachings of the most highly qualified publicists – are sources of evidence for the determination of rules of law on the basis of the formal sources listed in Articles 38(1)(a)-(c). These means are not sources of binding rules in and of themselves, but can provide evidence that the conditions set out in (one of the) formal sources have been fulfilled. This is confirmed by the reference to Article 59 ICJ statute, which states that the decisions of the Court have “no binding force except between the parties and in respect of that particular case”. As will be explained below, that judicial decisions are not binding as such does not mean that the jurisprudence of international courts and tribunals does not play an important role in shaping the international legal system.

C. Sources Beyond Article 38?Edit

I. Soft Law InstrumentsEdit

II. Unilateral DeclarationsEdit

D. Sources and Notions of Hierarchy in International LawEdit

At first sight, it may appear that the sources listed in Article 38(1)(a)-(c) ICJ Statute are listed in a specific order, denoting a hierarchy between them. This is not the case. There is no hierarchy between the sources of international law. This is separate from the question of hierarchical relationships between international norms. As will be discussed below, there are clear examples of such relationships, yet a rule will not prevail over another because of its source. The question of a hierarchy between the sources is also separate from the question of the role and importance of the different sources of international law more generally. For example, much of international law-making is done by states through concluding treaties. There are now thousands of bilateral and multilateral treaties covering a broad range of topics. As such, much of international law will be treaty law. Conversely, it may be possible to argue that many of the fundamental rules of general application are custom or general principles. Once again, these realities do not denote a hierarchy between the sources, but different roles played by the rules emanating from the different sources.

E. Concluding RemarksEdit

Further ReadingsEdit

  • Source I
  • Source II


  • Summary I
  • Summary II

Table of ContentsEdit

Back to home page

Part I - History, Theory, and Methods

Part II - General International Law

Part III - Specialized Fields


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