• What rules govern the formation of CIL? The conduct-centred model • The CIL approach to state behaviour: states as agents with legal motivations • What are the material sources of CIL? • Identifying particular rules of CIL: the tipping point • (Why) do states comply with CIL? • Conclusion CHAPTER OVERVIEW This chapter focuses on what customary international law (CIL) is and how we can identify it. CIL is an important source of international law as it is the only universally binding branch of public international law (PIL). It is also controversial because of this claim to universal application. This chapter first briefly maps the theories of CIL and then turns its attention to the dominant, conduct-centred model of CIL in international law. It then focuses on identification of particular CIL rules and how CIL evolves and changes.The chapter concludes with a discussion of how we can approach compliance with CIL.

2021

Customary International Law has been defined under Article 38(1)(b) of the International Court of Justice Statute as a “usual and general practice that is accepted as a law”. However, with the increase in the number of States, it is essential to establish a uniform practice all over the world. Therefore, codification of customs becomes vital in today’s world. The traditional elements of Customary International Law have not vanished, but they have been redefined to an extent and their importance reevaluated.

2019

Interpretability of customary (international) law belongs to the class of jurisprudential problems that entangle and intertwine almost all thorny theoretical and practical issues. It is especially visible against the background of debates around whether customary international law (CIL) can be interpreted, and if so, how this differs from its identification; are there or should there be some rules of interpretation of CIL, and what would be the difference between such rules and those guiding interpretation of treaties, etc. This contribution aims at addressing some of these issues. It seeks to suggest a meaningful way of seeing the process of interpretation of CIL through the perspective of practical reasoning. By doing so, the article purports to disentangle one of the theoretical knots of interpretation of CIL: what is the difference between interpretation of rules of CIL and their identification, considering that both processes concentrate on state practices. The first section will address the issue of duality of CIL within the doctrine of the container/content distinction. Section 2 will suggest a view of state practices as inherently normative, whereas section 3 will provide a more detailed analysis of customary normativity. The final section will highlight the difference in interpretation of state practices depending on their container/content perception and will therefore attempt a differentiation between the interpretation for the purpose of identification and interpretation for the purpose of clarification/application of a rule of CIL.

Pravni zapisi, 2021

The main objective of this paper is to critically assess the dominant additive theory of the formation of Customary International Law by using the concept of discursive normative practice and the work of Gerald Postema. My central conclusion is that the use of this concept provides an explanation of the process of formation of Customary International Law that is superior to the additive theory which consists of two elements - practice and opinio juris. On the other hand, Postema's theory also has its own weaknesses, and this paper explores ways to improve it.

International Community Law Review

Over the last few decades, the methodology for the identification of customary international law (cil) has been changing. Both elements of cil – practice and opinio juris – have assumed novel and broader forms, as noted in the Reports of the Special Rapporteur of the International Law Commission (2013, 2014, 2015, 2016). This contribution discusses these Reports and the draft conclusions, and reaction by States in the Sixth Committee of the United Nations General Assembly (unga), highlighting the areas of consensus and contestation. This ties to the analysis of the main doctrinal positions, with special attention being given to the two elements of cil, and the role of the unga resolutions. The underlying motivation is to assess the real or perceived crisis of cil, and the author develops the broader argument maintaining that in order to retain unity within international law, the internal limits of cil must be carefully asserted.

This is a brief introduction to a special journal issue on international customary law. The note offers information on the studies published in the issue and identifies two (interlinked) threads that connect them, namely state-centrism in custom making and the role of state will/consent (associated with the voluntarist school of thought within international legal positivism). Moreover, the note links the papers published in the issue with the work of the ILC on the identification of international customary law and argues that, to a certain degree, while offering authoritative guidance on the identification of custom, the ILC defines customary international law as a source of law. This, in a sense, overlaps with the function of judges/courts, which also define custom as a source of international law when they employ that source as a means to identify a customary rule. This means that ILC's approach on customary law may limit the power/authority of judges/courts to construct their own definition of custom as a source. The question to be asked then is how flexible or rigid should ILC's definition of customary law be.

The American Journal of International Law, 2005

Customary international law (CIL) is under attack as behaviorally epiphenomenal and doctrinally incoherent. In this article, we reject both claims. To be sure, CIL is a feat of levitation; it rests not on a rock-solid natural law basis of divine principles, but on a fabric of rational acts, woven through a multiplicity of relations over time. And while there are limits on, and variations in, the effectiveness of CIL, we argue that there are circumstances where it may independently affect the behavior of states. There is no reason in theory, or in data adduced by others, to believe CIL to be generally epiphenomenal. Since certain components of CIL serve as the foundation of all international law, this article suggests the circumstances under which one would expect international law to affect state behavior.

The paper examines the legitimacy of customary international law on three merits: Legal, Moral, and Social Legitimacy. The paper starts with the examination of the definition of legitimacy. Considering the difficulties in the creation of a unified definition the paper divides the legitimacy into the three mentioned merits. The paper concludes that CIL in certain circumstances can be considered as legally illegitimate as it breaches the notions of consent and sovereignty. However, CIL is morally and socially legitimate due to its benefits for the development of international law and overall popular acceptance.

Journal of Community & International Law, 2023

In international law, it is common for customary international law that predates a treaty on the same subject to continue to govern alongside the treaty. Sometimes, the rules of a treaty can evolve into CIL if non-parties accept those rules as law. However, this process typically excludes the behaviour of treaty parties, who follow the rules due to their treaty obligations. This article explores a notable exception to the ability of nonparties to shape CIL from treaty rules. It argues that when a treaty is endorsed by a vast majority of States, the influence of the small minority of non-parties is diminished. This majority effectively overrides pre-existing CIL (if inconsistent with the treaty), establishes a new CIL through broad consensus, and confines future CIL within the treaty's framework, eliminating the possibility of dissent.