When faced with the inevitable task of interpreting customary international law (CIL), what should a court do and what should it consider? Should it engage in an ‘inductive’ process of sifting through available evidence of state practice and opinio juris or a deductive process designed to reason logically from principles embedded in the rule? Should the court invoke something like the rules of treaty interpretation with their focus on good faith, ordinary meaning, context, and object and purpose? International law doctrine falls short here. Figuring out how to interpret and apply custom requires a theory of custom, a focus on the normative stories we tell. This reveals that there is not just one story explaining why custom should be a source of law, but multiple stories. What we call ‘custom’ may represent or draw from at least three different sources of law: Negotiated Law, Legislated Law and Adjudicated Law. This chapter aims to show that the non-treaty rules derived from each draw on different sources of legitimacy, operate according to different logics, dictate different methods of interpretation, and favour different methods for resolving disputes.

This chapter examines the unparalleled influence that the Court’s decisions have had on the Commission’s codification and progressive development of areas of the law under its consideration. It illustrates not only the great extent to which many of the Commission’s propositions have borrowed their authority from the pronouncements of the Court, but also the significant impact of the latter on the Commission’s choices concerning terminology and programme of work. The chapter further demonstrates the Commission’s conscious efforts to support the Court’s cause more broadly, including by encouraging the expansion of the Court’s jurisdiction and by promoting the doctrine of the sources of international law enshrined in its Statute.

Chapter 2, ‘Finding Support for Indigenous Peoples’ Participation in the Sources of International Law’, turns to a doctrinal analysis. It undertakes an examination of relevant sources of international law on self-determination and Indigenous peoples’ rights, in treaties, declarations, and decisions of international judicial and quasi-judicial bodies. We see how the law of self-determination has been interpreted by the latter bodies as meaning – among other things – participation of peoples at the national (but not necessarily the international) level, and how participation is at the heart of the law on self-determination specific to Indigenous peoples. The chapter then turns to customary international law, reviewing and contextualizing various methodologies for its identification and summarizing how the evidence described in later chapters can be interpreted through these methodologies. Through these lenses, I discuss methodological debates, including the legal status we should assign to the UNDRIP in and of itself, how a provision of the UNDRIP might later crystallize into a rule of custom, and how to regard international organizations in relation to the identification of custom.

The author attempts to answer the following question: in an international legal system of which States are no longer the only subjects, what is the role that the consent of IOs plays in the creation of rights and obligations that apply on the international plane? The chapter reflects on the legal agency that IOs enjoy, qua subjects of international law, in the process of formation of general international law and in the acquisition of treaty rights and obligations. In so doing, it advances two claims. The first is that even though IO practice can and should be taken into account in the identification of customary rules, it is not a necessary element in the process of formation of those rules. The second claim of the chapter is that, under current law, IOs can only be bound by treaty rules to which they have consented, which may give rise to problematic gaps between the treaty obligations of IOs and the treaty obligations of their members. In pursuing those claims, the chapter offers some reflections on the systemic and normative implications of the formal involvement of IOs in the making of rules of general and conventional international law.

This chapter explains the Court’s significant role as the ‘master of the sources’ of international law. The author considers what he defines as the ‘repressive dimension of Article 38’, before turning to consider the ways in which the Court has concretely used its role in respect of the sources of international law. Finally, the chapter critically examines how repression and mastery can often work together in international legal thought.

This chapter expands on the concept of reciprocity, looking at how it can be defined, its key characteristics, and the three key functions it plays: in norm creation, as a condition, and in execution of the law. It examines the defining elements of reciprocity: proportionality, relativity, equality, and interdependence. The chapter goes on to examine the nature of reciprocity, and its relationship to custom and general principles, two sources of international law. It illustrates how it is reciprocity’s relationship with the structural characteristic of sovereign equality in international law that explains many of its roles and defining characteristics.

This chapter introduces the concept of foreign direct investment, tracing its history and economic justifications. The chapter goes on to explain the sources of international investment law, the most significant of which are treaties.

What is international law, and how is it different from domestic (national) law?  This chapter provides an answer to those questions and introduces other foundational concepts in international law such as opinio juris, sovereignty, and the state. We trace the development of international law from Western/European perspectives and discuss how international law was often experienced as oppression and violence in the non-Western world. The chapter concludes with a consideration of the classic sources of international law: customs, treaties, general principles of law, judicial decisions, and scholarly writings, each of which is presented in some detail.

The story finally gets to the interpretation and application of legal norms to the facts of the case. This chapter deals with the first interpretive step, called law-ascertainment, where the interpreter identifies the rules that are relevant to the case at hand. Donning the hat of an explorer, the interpreter maps the body of norms precedents in concentric circles – starting with the sources closest to the centre of gravity of their judicial regime and gradually venturing into more peripheral territory. The results of these practices explains the degree of cohesion or fragmentation in international law. As the legal system grows more complex, interpreters no longer have time and resources to master it all, and prove increasingly partial and selective in their legal readings. The emergence of self-contained regimes is largely due to epistemic constraints which, in turn, reflect the social structures of the international judicial community.

The EU functions as a receptor and promoter of customary international law (CIL). However, from a theoretical (and international law) perspective, the questions why and when the EU is bound by CIL still have not been answered in definite terms. This chapter argues that the EU should be afforded a similar treatment as is afforded to (newly independent) states. This is based on a close examination of its practice in the field of international humanitarian law (IHL), where the primary organization tasked with ensuring peace and security – the United Nations – has so far avoided explicitly affirming that it is bound by customary IHL. After showing that it is indeed the EU which might bear direct responsibility in the context of military operations authorized under the Common Security and Defence Policy, the chapter examines the EU’s behaviour when it comes to new regulatory efforts, its executive and operational conduct, and the CJEU’s case law. On the basis thereof, it is concluded that the EU’s practice confirms the theoretical conclusions reached, and that therefore any theoretical uncertainties on whether and why international organizations are bound by CIL might in future be answered by paying closer attention to their practice.

The scholarship on sources of international law has not satisfactorily answered these questions: are sources regulated by any ‘rules’? Are ‘rules on sources’ part of international law? What is the source of ‘rules on sources’? The standard answer to these questions is that sources are regulated by state practice reflected in Article 38(1) of the Statute of the International Court of Justice. This position, albeit professing to be practice-based, does not account for the existence of ‘rules on sources’, nor for their legality and source-based character. Other practice-based positions have also proven insufficient: some, postulating the legality of rules on sources, fail to address their source-based nature; others, affirming both the legality and source-based character of rules on sources, and invoking custom as source thereof, fail to substantiate their claims, which are merely asserted or based on decisions of early arbitral tribunals, thus assuming custom to be judge-made (a ‘custom in foro’). This chapter examines the suitability of custom as source of rules on sources and proposes a shift away from custom in foro towards customary international law proper, established by state practice invoking ICJ Statute Article 38(1) outside ICJ proceedings, major instances of which it surveys.

Chapter one starts from my embarrassment when teaching sources of international law. Following conventional wisdom, I inform students that international law is grounded on a limited set of sources. However, at some point, I also have to explain that it is possible for new sources of international law to emerge. How is this possible, given that international law is grounded on a limited set of sources? I try to deal with this uneasiness by comparing discourses on sources to rituals that prevail in what I call "cyclical societies," organized around the belief in the eternal return of transcendental ideas, acts or events. To apply sources, I argue, is to perform a double act of repetition. First, historically contingent events are turned into manifestations of pregiven and repeatable categories. Second, sources are used as placeholders for something that will always escape positive international law: the foundational categories that underlie the sources of law. These foundational categories, I argue, work somewhat like celestial Gods in cyclical societies: Most of the time they stay dormant and aloof, but they can always be called upon in exceptional times.

This chapter analyses the ICJ Statute Article 38(1), interpreting each its component terms, with a particular emphasis on ‘teachings’. One overall conclusion is that the wording of Article 38(1) gives at best limited on guidance on the application of teachings, which supports the utility of the examination made in the rest of the book. When interpreting ‘teachings’ specifically, it is particularly difficult to classify works produced by the International Law Commission. They are not seen as ‘teachings’ in the book.

Two main themes of the disicipline of international law, namley its theory of subjects of international law and the concept of sources of international law form the core of this chapter. Respective views of Hobbes and Leibniz on these two themes are analysed and compared. The continuing influence of Hobbesian and Leibnizian ideas on these two topics is also analysed. It is argued that the theory of subjects of international law with the central role of the state and the objective definitional approach to the concept of the state is fundamentally Hobbesian. The concept of sources of international law appears at the surface more Leibnizian. However, the Leibnizian heritage was striped of its foundation and thus distorted. As a result, international law in its contemporary articulation is an oxymoron. The trasformative future of international law lies in a rethinking of its normativity away from its Hobbesian heritage. However, to be sucessful, such a rethinking should be grounded in a reflection on possible alternative spatial-conceptual foundations one of which is offered by Leibniz.