Part I: Unity, Diversity, and Evolution of Interpretation across Sources of International Law

A force that drives the evolution of interpretation in the practice of international law is its multifarious and interdependent relationship with the rules comprising the theory of sources of international law. In the context of treaty interpretation, the VCLT rules coexist alongside methods, maxims, and practices that might not fit neatly into the means and methods of interpretation envisaged in the VCLT or might expose real or apparent blind spots in those rules. Equally, the fact that interpretation is a process legally and analytically distinct from the process of identification does not mean that it operates in clinical isolation from the broader theory of sources of international law.Footnote ¹⁶ Interpretation presupposes identification: how practice approaches one process necessarily has ripple effects on the other. On the one hand, interpretation has the capacity to alleviate (or amplify) the challenges posed by the piecemeal formation of international law, such as norm-conflict or legal incompatibility or uncertainty.Footnote ¹⁷ On the other hand, differences in the process of identification of rules of international law, like their unwritten nature, might militate for differentiation in terms of the process of interpretation as well, such as the unavailability of certain means of interpretation.Footnote ¹⁸ The contributions in this part draw upon these broader themes.

In the opening contribution of this part (Chapter 2), Jure Vidmar reflects on the role of general principles of law, jus cogens, and erga omnes obligations in the practice of interpretation of international law. Vidmar starts from the premise that international law formation through treaties and customary international law is inherently based on a private law paradigm characterised by bilateralism and consensualism. For the most part, this private law paradigm permeates into the process of interpretation as reflected in Articles 31–33 VCLT. That said, the principle of ‘systemic integration’ enshrined primarily in Article 31(3)(c) VCLT and its customary counterpart presumes a systemic structure of international law. In this context, obligations erga omnes, jus cogens norms, and, to some extent, general principles of law provide for at least a rudimentary public law framework, which dilutes consensualism and enables a certain degree of systematisation of treaty and customary law. Such systematisation then takes places through the process of interpretation and, more specifically, systemic integration. Underlying Vidmar’s analysis is the claim that the rule of interpretation requires the systemic integration of the rules of international law ‘regardless of the source from which these rules may be stemming’.Footnote ¹⁹

In Chapter 3, Yannick Radi similarly examines practices of interpretation across different sources of international law, with a focus on the interpretation of international investment agreements (IIAs). In this chapter, Radi posits that international investment law is a regime in search of its identity. At the core of this search, a central role is played by ‘comparativism’ (or comparative reasoning) as a method of interpretation employed by investment tribunals. The chapter explores how comparative reasoning fits into the theory of sources and how this method is used by arbitral tribunals to convince their audience. Radi finds that arbitral tribunals regularly rely on customary international law, general principles of law, as well as previous judicial decisions, when interpreting IIAs provisions. However, in doing so, arbitral tribunals display a selectivity in the choice of domestic rules or systems considered for this purpose. Relatedly, when relying on previous judicial decisions, they do not always enquire sufficiently into the similarity (or lack thereof) between the past judicial decisions and the present issue at hand. As these trends raise issues from the standpoint of legitimacy, Radi observes that arbitral tribunals rely on comparative reasoning to strengthen or justify the choices made in the course of interpretation.

Finally, in Chapter 4, Ilias Plakokefalos focuses on the interpretation of customary international environmental law (IEL) before international courts. Through a study of the jurisprudence of the International Court of Justice (ICJ), Plakokefalos shows that the range of methods that courts can use for the interpretation of customary law is in principle no different than that applicable to other sources of international law and that these methods roughly correspond to those enumerated in Articles 31–33 VCLT. At the same time, the question is raised whether all these methods are suitable to the interpretation of customary rules of IEL. Plakokefalos submits that it is difficult, and perhaps even fruitless, to interpret customary environmental rules by reference to their object and purpose, given the multiple (at times even contrasting) objects and purposes that these rules can have. This is the main reason why most environmental rules have by their inception an inherent constraint with regard to teleological interpretation as they yield to a number of considerations beyond simply the protection of the environment. Against this background, Plakokefalos finds that international courts and tribunals have interpreted customary environmental rules in expansive as well as regressive ways, oscillating between these two tendencies. When interpreting expansively, courts take into account developments that appear to affect the rule in question and push it to an interpretive point where it catches up with these developments. In contrast, regressive interpretation involves а backward looking approach where the courts are content with offering an interpretation that diverges from the standards surrounding the rule and renders a more conservative version of it.

Part II: Unity, Diversity, and Evolution of Interpretation of International Law across Actors

It is often said that the best way to learn about the art of interpretation in international law is to look at the practice of international courts and tribunals.Footnote ²⁰ Undeniably, the practice of international courts and tribunals, especially the Permanent Court of International Justice (PCIJ) and the ICJ, has been instrumental for the codification of rules of interpretation for treaties and unilateral acts.Footnote ²¹ In this context, one of the most theorised aspects of the formal approach towards interpretation in international law has been the extent to which such rules can have a limitative function, that is, influencing actors to arrive at certain interpretative conclusion.Footnote ²² However, an often neglected aspect of rules of interpretation in international law is their enabling function: they specify how international law will be interpreted without posing any restrictions as to who will interpret them.Footnote ²³ As a result, a variety of actors including international organisations, expert bodies, or non-governmental organisations can raise claims about the meaning of international law and, in this way, contribute to the development of international law.Footnote ²⁴ The contributions included in this part zoom into the neglected interpretative practice of actors besides international courts and tribunals. How do these interpretative practices feed into the evolution of the process of interpretation, and do they drive it towards unity or diversification?

This part kicks off with Chapter 5 by Catherine Brölmann, dedicated to the interpretation of customary international law (CIL) in an institutional context. The ‘institutional context’ in this chapter refers to the institutional environment of an international intergovernmental organisation, and the chapter focuses on the interpretation of CIL by international organisations. Brölmann maps all the different scenarios when the institutional context is relevant in the interpretation of customary international law, including: (i) instances of an organisation applying and interpreting CIL on par with other international actors; (ii) instances when the interpretation of organisations contributes to the formation of customary law; (iii) instances when international organisations apply and interpret CIL within their institutional order; and (iv) instances when soft law produced by international organisations co-creates customary international law. Based on the overview of these different scenarios, Brölmann draws a number of tentative conclusions about the interpretative practice of international organisations. Firstly, the separate institutional sphere of an organisation adds a layer of normativity and authority next to that of general international law. Therefore, it may happen that instruments created by the organisation would have normative effect within the institutional framework but not per se in general international law. Conversely, a rule of customary international law could be of uncontested validity but not have automatically normative force within the institutional framework. Secondly, within the institutional framework, conduct of the organisation may not be readily distinguishable from the conduct of member states. This may then influence the identification of potentially new customary rules. Finally, it seems that the institutional context is conducive to the teleological method of interpretation. Brölmann traces this trend both in the interpretation of constitutive treaties and customary rules.

In Chapter 6 by Ellen Polcinski, we find an overview of the interpretative practice of yet another international actor – the International Committee of the Red Cross (ICRC). In her chapter, Polcinski positions the ICRC as the ‘guardian’ of international humanitarian law and treats the Commentaries to the Geneva Conventions produced by the ICRC as authoritative guidance on how these treaties are to be interpreted. Thus, in Polcinski’s analysis, the ICRC represents an actor whose interpretation is authoritative on its substantive merit. Chapter 5 studies the interpretative methodology behind the Commentaries of the ICRC, eventually concluding that although the methodology has evolved, it has done so apace with the evolution of the rules of treaty interpretation in general public international law – culminating in the adoption of the VCLT. Polcinski considers this as decisive proof that the rules of international humanitarian law (IHL) are subject to the same interpretive rules as other international rules. A further observation that may be drawn from her analysis is that the use of the general rules of interpretation by the ICRC – an actor who is not formally bound by these rules – points to a broader systemic feature of interpretation in international law.

Continuing in a similar vein, Chapter 7 by Marlies Hesselman analyses the interpretative practice of Human Rights Bodies (HRBs). HRBs in this context refer to the human rights supervisory bodies established by the nine core human rights treaties. Their interpretative practice considered in the chapter includes the consideration of individual or inter-State complaints, periodic State reports, and the more general guidance provided by these bodies on the content of treaty norms (e.g. General Comments). At the outset of her analysis, Hesselman notes that when engaging in interpretation, HRBs may follow specific methods of treaty interpretation – including those derived from Articles 31–32 VCLT – or not. However, Hesselman’s analysis goes one step further and examines how the interpretative pronouncements of HRBs are themselves considered as authoritative in the subsequent interpretation of treaties or other sources of international law. This is done under the framework of ‘pronouncements of expert bodies’, as examples of either subsequent practice in the interpretation of treaties within the meaning of Articles 31–32 VCLT, or as ‘subsidiary means’ for the determination of rules of international law in the sense of Article 38 ICJ Statute. Hesselman concludes that, given the expertise and volume of interpretative pronouncements of HRBs, they need to be taken seriously when it comes to the determination of content of human rights rules. However, at present it is not yet clear whether these pronouncements can be relied on as means of interpretation via the formal avenues available in international law.

Finally, Part II concludes with Chapter 8 by Elisa Morgera, dedicated to the collective interpretative practices of non-judicial actors. In her analysis, Morgera relies on the example of the One Ocean Hub, an inter- and transdisciplinary ocean research network that engages academic and non-academic stakeholders, including local decision-makers and representatives from international organisations, civil society, and affected communities, to co-develop mutually supportive interpretations of international environmental law, the law of the sea, and international human rights law. Morgera argues that the interpretative practice of the One Ocean Hub related to the protection of human rights that are dependent on a healthy ocean is an example of collective interpretation whose authority comes from the expertise of the involved practitioners. She examines how this interpretative practice can be institutionalised under the law of treaties, with a view to embedding it in the work of relevant treaty bodies.

Part III: Unity, Diversity, and Evolution of Interpretation across Subject Matters of International Law

A necessary corollary of a formal approach to interpretation is that rules of interpretation, like any other legal rule, are amenable to evolution by being themselves subject to interpretation, amendment, or displacement by other rules of interpretation. The diversification of international legal practice into distinct areas of knowledge and expertise based on subject matter can potentially pose a destabilising effect. In the context of treaty interpretation, the almost unanimous acceptance of the VCLT rules in legal practice does not necessarily entail that they are the final or even clear word on the matter. On the one hand, the entrenchment of interpretative practices along the lines of subject matter expertise is often expressed as a distinctive understanding of the internal relationship between the means of treaty interpretation envisaged in Articles 31–33 VCLT.Footnote ²⁵ On the other hand, specific regimes might lay down special rules of interpretation that bring to the fore the external relationship of the VCLT with such special rules. This part draws upon these broader themes.

Part III opens with Chapter 9 by Gabrielle Marceau, Maria George, and Vishakha Raj, analysing the application of the general rules of interpretation in the Dispute Settlement System (DSS) of the World Trade Organization (WTO). In their contribution, Marceau, George, and Raj analyse the ‘principle of single undertaking’ as a unique feature in the WTO context and reflect on the effect of this principle on the interpretative practice in the WTO DSS. The ‘single undertaking’ here refers to the fact that the WTO treaty comprises several agreements all of which bind WTO members simultaneously and cumulatively, thus requiring implementation as a single undertaking. Marceau, George, and Raj argue that to maintain coherence within this single undertaking, the WTO DSS has engaged in horizontal cross-fertilisation of jurisprudence across agreements. This, they conclude, is done by reliance on the tools and methods of treaty interpretation as defined by the VCLT rules of interpretation and their customary counterparts.

Continuing in a similar vein, Chapter 10 by Thomas Nektarios Papanastasiou focuses on the interpretation practice of international investment tribunals, using the territoriality requirement in international investment arbitration as a case study. Papanastasiou identifies various interpretative methods that international investment tribunals employ when interpreting the scopes ratione loci and ratione materiae of investment treaties. In doing so, he finds that on many occasions arbitral tribunals use methods interpretation that favour the object and purpose of treaties over other elements of interpretation envisaged in Articles 31–33 of the VCLT. When applied to the interpretation of the territoriality requirement, Papanastasiou finds that arbitral tribunals interpret the requirement based on the intention of the contracting parties to create favourable conditions for the promotion of investments and economic cooperation. Such an interpretive approach creates an expansive tendency of the territorial application of investment treaties.

In Chapter 11, Olympia Bekou turns to the examination of international criminal law and the interpretative practice with respect to the Rome Statute of the International Criminal Court (ICC). Interestingly, in Bekou’s chapter, interpretative practice refers to the adoption of national legislation implementing the Rome Statute in national legal orders. In the process of national implementation, Bekou argues, states offer their interpretation of the Rome Statute. Her analysis is then focused on the interface between national implementation as interpretation by states and the interpretation of the Rome Statute by the ICC. In particular, Bekou examines instances when these two interpretations of the Rome Statute diverge, with a view to determining how divergent interpretations by states might influence the way the ICC interprets the Rome Statute. As an international treaty, the interpretation of the Rome Statute is governed by the rules of interpretation as enshrined in the VCLT. Against this background, Bekou identifies several different ways in which the divergent interpretations of the Statute by states might need to be taken into account by the ICC when it interprets the Statute (e.g. as subsequent agreement within the meaning of Article 31(3)(b) of the VCLT). Ultimately, Bekou concludes that while it remains necessary to distinguish between an interpretation within the scope of the existing law and modification of the text of a treaty, judges of the ICC may rely on the VCLT rule of interpretation in order to account for the interpretative practice of states.

Finally, in Chapter 12 Leila Sadat reflects on the development of crimes against humanity in international law and the interpretative approaches of states in this process. Sadat provides a detailed genealogy of the development of the crime, from its nineteenth-century natural law origins through its codification in the Rome Statute and finally its possible recodification in a new treaty. She then traces the interpretative approaches of states towards the current draft proposal about a specialised treaty, ultimately concluding that states’ attitudes may fall into one of three categories – unity, evolution, or diversity. Unity refers to states who advocate for the new treaty to be a horizontal add on to the Rome Statute, making the two instruments consistent as a matter of text and application. Evolution refers to states who advocate for the progressive interpretation of existing provisions, with a view to adjusting the definition of crimes against humanity consistently with new development or expanding the list of crimes that fall in this category. Diversity, finally, refers to the possibility of states to continue developing the law even after codification, through evolutions in customary law or diversity in national codification.

The overall volume concludes with a contribution by Judge Ineta Ziemele of the Court of Justice of the European Union, reflecting on the theme of unity, diversity, and evolution in the practice of interpretation in international law. This contribution is an adjusted transcript of the keynote speech Judge Ziemele gave at the Hague Conference. In her contribution, Judge Ziemele reflects on the systemic function that interpretation performs in international law. International law, like any other legal system, must ensure legal certainty. This task is all the more important for international law in light of the constant criticism advanced towards the nature of this legal system. Thus, Judge Ziemele argues, one of the key tools to ensure legal certainty is a consistent and coherent application of rules based on a method of interpretation, where that method serves as the backbone of a judgement. The legal system and an act of application of a specific rule gain persuasiveness and legitimacy where the use of the method of interpretation of a rule is consistent from one case to another. Indeed, it is against this background that the overall theme of this volume becomes even more important. While the various contributors to this volume reflect on different actors as interpreters in international law, Judge Ziemele’s contribution zooms in specifically on judicial interpretation. Judges, she argues, are the guarantors of the resilience of law in the sense that it is in their power to ensure its impartial and uniform application, which in turn counters allegations of inefficiency or bias of that system of law. All these considerations about the systemic features of interpretation and the role of judges as interpreters play towards at least a perception of the rule of law in the legal system. Therefore, Judge Ziemele concludes, the general rule of interpretation of international law is a central element in the international rule of law.