Public International Law/Sources of International Law





Author: Craig Eggett

Required knowledge:

Learning objectives: to understand the nature of sources in international law; to understand the relationship between the sources. = Introduction =

The topic of the ‘sources’ of international law is essentially concerned with one central, and rather basic, question: how are international rules made? Despite, or perhaps because of, the foundational nature of this question, there are few areas of international legal scholarship that have generated such long-running and vociferous debate. Questions about the sources of international law have always been central to international legal discourse and understanding the language of the sources remains critical for all actors wishing to engage with the international legal system. The chapters that follow explore the different ways in which international legal rules are created and, in doing so, they reflect on the theory and practice of the sources of international law. This chapter introduces some broader questions about the sources of international law, with the aim of setting the scene for the examination of the individual sources that follows. To this end, this chapter proceeds in four main parts. First, it considers in greater detail the concept of a ‘source’ in international law. Second, it examines the content of article 38 ICJ Statute. Third, it considers the reality of the operation of the sources in practice. Fourth, it explores questions of hierarchy in the sources and norms of the international legal system.

= The Concept of a ‘Source’ of International Law =

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.

Sources and the International Legal System
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.

Categories of Sources?
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 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 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.

= Article 38 ICJ Statute =

Article 38 of the 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:

  international conventions, whether general or particular, establishing rules expressly recognized by the contesting states;   international custom, as evidence of a general practice accepted as law;   the general principles of law recognized by civilized nations;   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. 

  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 lex arbitri (Latin: ‘applicable law’) 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 article 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.

The reference to ex aequo et bono (Latin: ‘according to the right and the good’) in article 38(2) identifies the possibility that a dispute before the Court may be settled on the basis of equitable considerations, should the parties agree. This is a reference to a specific form of equity free from interaction with legal norms. To date, this provision has never been invoked before the ICJ.

= The Reality of International Law-Making =

There is extensive scope for rich theoretical discussion on the sources of international law. As with all areas of the discipline, such discussion is critical if the international legal system is to be able to achieve its objectives. Regardless of the position taken on the exhaustiveness of the article 38 or the role of state consent, this provision alone cannot paint an accurate picture of the practice of international law-making. International law is a decentralised legal system comprised of a plurality of actors and legal regimes. As a result, the sources of international law function in a dynamic manner, relying on a complex web of interactions between different bodies – such as states, international organisations, and judicial bodies – and reliance on a range of instruments, such as treaties, various decisions, and other ‘soft law’ instruments. This is only partly captured by the reference to judicial decisions and teachin of publicists in article 38(1)(d) ICJ Statute.

Example: The ILC and the Development of International Law

The work of the ILC has been instrumental in many areas of international law. The Commissions work has serves as the basis for treaties, including the VCLT, and its conclusions are frequently cited with authority. For example, the product of the ILC’s work on state responsibility (ARSIWA) – itself a comprehensive review of state practice, judicial decisions, and an array of other instruments, was confirmed to be part of customary international law but the ICJ in the Bosnian Genocide case.

= Hierarchy in the Sources of International Law =

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. There can be multiple rules that have similar or identical content, but emanate from different sources of international law. For example, in the Nicaragua case, the Court confirmed the parallel existence of customary and treaty rules regarding the use of force. This question of a hierarchy between the sources is separate from that of hierarchical relationships between international norms. While there are clear examples of normative hierarchies, 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 in different areas of international law. Broadly speaking, there are three aspects of international law that are referred to as evidence of hierarchical relationships between norms: article 103 UN Charter, jus cogens (Latin: ‘peremptory norms’) and obligations erga omnes (Latin ‘towards all’).

Article 103 UN Charter reads: ‘In the event of a conflict between the obligations of the Members of the United Nations under the present Charter and their obligations under any other international agreement, their obligations under the present Charter shall prevail’. It may be argued that this provision functions as a ‘supremacy clause’, elevating the UN Charter to a hierarchically superior position in the international legal system. While at first sight this seems to be the case, it should be noted that the practical effect of this provision is largely limited to the obligation to comply with UN Security Council resolutions contained in article 25, as there are few other specific and concrete obligations in the Charter. It should also be borne in mind that this provision functions more as a rule of precedence, very different to the consequences of a norm’s jus cogens status, for example.

Jus cogens norms are clear example of hierarchy in international law. These peremptory rules of international law are defined as rules ‘from which no derogation is permitted’. In the event of a conflict between a rule of jus cogens and another international rule, the jus cogens rule prevails and the other rule is void. Further, articles 40 and 41 of ARSIWA impose additional obligations on states in the event of serious violations of jus cogens norms, including a requirement to cooperate to bring about the end of the jus cogens violation and an obligation not to recognise as lawful any situation created as a result of such a violation. Established jus cogens norms include the prohibitions on genocide, slavery, torture, and racial discrimination.

Obligations erga omnes are defined as those owed “towards the international community as a whole”, with the result that “all [s]tates can be held to have a legal interest in their protection”. This seems to confirm the importance of such obligations, yet this concept does not imply a hierarchy between these and other norms. The label erga omnes serves to denote only an expansion in potential scope of actors who can invoke violation of the rule. This is a purely procedural device, which facilitates the enforcement of international rules which may not necessarily involve an injured state or to increase the likelihood of enforcement of rules deemed to be substantively important. Obligations erga omnes do not prevail over other rules of international law in the same way as jus cogens rules.

Advanced: The Relationship between Jus Cogens and Obligations Erga Omnes

The concepts of jus cogens and erga omnes are frequently taken together. Traditionally, it has been accepted that all jus cogens rules also have erga omnes status, but that the reverse was not necessarily true. Indeed, it is clear that both concepts serve to enhance the operation and level of protection for certain fundamental norms of international law. In the ILC’s recent work on jus cogens, there is further evidence of the close relationship between these two concepts. Here, the ILC suggested that the right to self-determination has also attained jus cogens status yet, in doing so, referred to ICJ decisions where only the erga omnes nature of self-determination was confirmed.

= Conclusions =

This chapter has explored the foundations of the doctrine of the sources in international law. In doing so, it has explained that the primary objective of this doctrine is to distinguish between rules that are part of the corpus of international law and those that are not. It has been explained that article 38 ICJ Statute constitutes an essential starting point for an account on the sources of international law, yet it does not paint a full picture. The relationships between international norms and their sources is complex and will be taken up further in subsequent chapters.

= Summary =


 * Sources of international law help to identify what the law actually is.
 * Article 38 ICJ Statute provides a starting point for the sources of international law, listing treaties, custom, general principles, and subsidiary means.
 * The development of international law is a dynamic practice, involving a range of actors, yet states remain at the centre of this practice.
 * There is no hierarchy between the sources of international law. Yet, there are some examples, particularly jus cogens, of hierarchical relationships between international norms.

= Further Reading =


 * S Besson and J d’Aspremont, The Oxford Handbook on the Sources of International Law (Oxford: OUP, 2017).
 * H Thirlway, The Sources of International Law (Oxford: OUP, 2014)