Revisiting international legal positivism: validity, legitimacy, and systematicity in international law

Abstract

This thesis revisits and reassesses one of the most influential yet controversial schools of thought in international law: international legal positivism. For decades, this school has profoundly shaped how scholars and practitioners understand the foundations of international law—what determines its existence (validity), why it is morally binding (legitimacy), and how it functions as a legal system (systematicity). Yet, despite its enduring influence, international legal positivism is arguably also the most heavily criticised theory. As Kammerhofer observes, international legal positivism is as dead as it is all-pervading. Such a paradoxical reputation of international legal positivism inspires and drives the present inquiry.

To unravel this paradox, this thesis revisits international legal positivism and asks the following questions: What are the core claims of international legal positivism? Where do those claims—or our perception of them—go wrong? And what implications do they hold for international law today? In response, this thesis identifies and disentangles two conflated claims at the heart of international legal positivism: the 'social facts approach' to validity, which asserts that the existence of international law depends on social facts rather than its moral merits; and the 'voluntarist approach' to legitimacy, which grounds the legitimacy of international law in state consent. While existing scholarship seems to neglects the relationship between these two claims, this thesis argues that they are not merely two parallel claims under the banner of international legal positivism but are substantially conflated. In particular, there appears to be a prevailing perception that the 'social facts approach' to validity necessarily goes hand in hand with the 'voluntarist approach' to legitimacy, leading to a comprehensive 'state will approach'. It is this substantial conflation, the thesis argues, that lies at the core of contemporary (mis)understandings of international legal positivism, and profoundly shapes the way international law is debated and practised. It is also this conflation that renders international legal positivism both conceptually confusing and normatively contentious.

The first part of the thesis explores the history of this substantial conflation. It observes that, originally, international legal positivism—much like its counterpart in analytical jurisprudence—focused primarily on the 'social facts approach' to validity. However, Lassa Oppenheim's normative engagement with John Austin in the early 20th century marked a turning point. Oppenheim responded to Austin's conceptual critique—that international law is 'law improperly so called'—from a normative perspective: he shifted the debate from validity to the legitimacy of international law, and even endorsed voluntarism as its justification. As probably the most representative international legal positivist in the field of international law, Oppenheim's theory has significantly shaped the prevailing perception of what international legal positivism claims. In this sense, his normative turn was misleading and contributed to the perception that international legal positivism endorses a 'voluntarist approach' to the legitimacy of international law, in addition to its 'social facts approach' to validity.

Afterwards, this conflation has been further reinforced in several different ways. For example, some voluntarist theories were mislabelled and placed within a narrative that portrayed them as responses to Austin's alleged 'normative' critique, even though these theories were situated in, and written against, a different intellectual backdrop. Moreover, the conflation was reinforced by misplaced critiques of international legal positivism, which reproached it—more specifically, its voluntarist claim—for failing to provide a satisfactory account of legitimacy, instead of clarifying that it did not intend to address legitimacy in the first place. These aspects have reinforced the conflation between positivism and voluntarism, rather than disentangling it. (Chapter 3).

The thesis further argues that, after being included under the banner of international legal positivism, the 'voluntarist approach' has subsequently shaped how the 'social facts approach' has been interpreted in international law—particularly by narrowing it into what may be termed a 'state will approach'. Originally, 'state will'—used as an equivalent to 'state consent'—was invoked as a normative justification for the legitimacy of international law.

Over time, however, 'state will' was transplanted into discussions of validity and disguised as the 'social fact' under the name of international legal positivism. However, this transplantation was not justified through an explanation of how 'state will' could properly be regarded as the 'social fact' underpinning the validity of international law. Instead, it was achieved by mistakenly tracing this voluntarist element back to legal positivists in the analytical tradition, particularly John Austin. In the end, these two claims were merged into a comprehensive 'state will approach' to both validity and legitimacy of international law, under the name of international legal positivism. (Chapter 4).

The second part of the thesis examines the implications of this conflation. It argues that the substantial conflation of positivism with voluntarism has obscured discussions of both the validity and the legitimacy of international law. It has sparked controversy in, inter alia, doctrinal debates concerning the sources of international law, and has also given rise to challenges from critical perspectives, notably those advanced by Martti Koskenniemi.

For instance, in discussions of customary international law, the function of opinio juris in identifying customary international law differs significantly from the concept of 'state will'—which is transplanted from the context of legitimacy and essentially denotes 'consent to be bound'. However, the differences between opinio juris and state will have been strategically exploited by opposing camps in the debate. One camp reasons from validity to legitimacy: it starts from the nature and role of opinio juris in the context of validity, and thereby criticises the 'state will approach' to validity. Some of its supporters go further and reject the 'state will approach'—i.e. voluntarist approach—to legitimacy. By contrast, the other camp proceeds in the opposite direction, reasoning from legitimacy to validity: it begins with a 'state will' approach to legitimacy and then transplants 'state will' into the domain of validity, thereby secretly displacing opinio juris. As a result, the two camps ultimately ground their arguments in different elements (opinio juris and state will, respectively) as well as in different issues (validity and legitimacy, respectively) and thereby talk past each other, leading to an apparent impasse. (Chapter 5).

Another example can be seen in Koskenniemi's analysis of the concreteness–normativity dilemma, which, this thesis argues, is itself preoccupied with this conflation. This thesis unpacks the two binary oppositions in Koskenniemi's account—'concreteness' and 'normativity'—and argues that they in fact correspond to the 'social facts approach' to validity and the 'non-voluntarist approach' to legitimacy, respectively. In this sense, they do not necessarily constitute a dilemma, unless one presupposes the conflation that the 'social facts approach' to validity is necessarily intertwined with the 'voluntarist approach' to legitimacy, which thereby excludes non-voluntarism. (Chapter 6).

The final part of the thesis examines the theoretical payoffs of disentangling these claims and revisiting legal positivism on its own terms within international legal discourse. In particular, it explores the potential of Hartian legal positivism in addressing and understanding the systematicity of international law. Critiques and doubts about the systematicity—and the related issue of fragmentation—of international law have been longstanding. In this regard, this part argues that Hartian legal positivism provides valuable insight into the systematicity of law: it is not only about the existence and union of primary and secondary rules—which may be termed 'normative systematicity'—but also about the practices of legal institutions ('officials' in Hart's terms) in effectively producing and applying secondary rules—which may be termed 'functional systematicity'. This part focuses in particular on the 'functional systematicity' of international law under Hartian legal positivism, which seems to pose a greater challenge to its systematic character and, ultimately, to its effective functioning nowadays.

This part first examines how international legal institutions—which are increasingly pluralist, separate, yet overlapping—can operate together in a heterarchical structure. Notably, it adds to current discussions of Hartian legal positivism by arguing that this theory in fact supports and accommodates a broader understanding of 'functional systematicity'. Moreover, based on the interplay of 'normative systematicity' and 'functional systematicity', it enables a valuable framework for understanding how international law can function as a system without necessarily replicating the hierarchical structure of state law. Examined under this Hartian lens, this part thus offers a more nuanced understanding of how international legal institutions—i.e. the 'officials' in the Hartian sense—operate together within a heterarchical structure, especially through two mechanisms: (1) normative systematicity, which underscores the interdependence of international legal rules and prompts institutions to coordinate with one another, and (2) the role of epistemic authority, which further facilitates institutional interaction and coordination. (Chapter 7).

This part also examines another dimension of 'functional systematicity', namely, whether and how non-state actors can qualify as the 'officials' in the Hartian sense, capable of producing and applying secondary rules. Contrary to the critique that legal positivism is inherently state-centric, it argues that Hartian legal positivism in fact enables and supports a more pluralist, dynamic, and context-responsive understanding of who can qualify as the 'officials' for producing and applying secondary rules, one that can accommodate and explain the participation and contribution of non-state actors. (Chapter 8).

The last chapter supplements the theoretical analysis developed in the preceding two chapters with an empirical example, that is, the formation of rules of recognition of customary international law. This example shows that, despite the absence of a hierarchical structure akin to that found in domestic legal systems, the two mechanisms outlined above have contributed to institutional coordination and the eventual convergence of rules of recognition. More specifically, international legal institutions even appear to be arranged by a loose division of labour in their practice of such rules of recognition. This dynamic is conceptualised using Francis Bacon's metaphor of spiders, bees, and ants: the Permanent Court of International Justice (PCIJ) and the International Court of Justice (ICJ) typically act as the 'spider', weaving the foundational framework, which is widely cited and followed by other legal institutions. Other courts function as the 'ants', following and applying the approaches adopted by the ICJ, leading to the proliferation of such rules. Scholars— whether affiliated with academic institutions or acting individually—serve as the 'bees', processing, refining, and systematising these rules in judicial practices. (Chapter 9).

These arguments are not intended as a defence of the systematicity of international law.

Rather, it is acknowledged that international law is not yet perfect in this regard. Indeed, the above mechanisms do not yet operate successfully in all regimes of international law.

However, these limitations do not imply that reforming international law—or any other non-state legal system—must necessarily model itself on state law. In this regard, Hartian legal positivism provides a valuable framework for understanding how international law can function as a system without necessarily replicating the hierarchical structure of state law. Rather, based on the interplay of 'normative systematicity' and 'functional systematicity', it highlights the possibility that governance can emerge from alternative structures, such as a heterarchical one in the absence of a supreme and centralised authority.

Therefore, viewing the contemporary international legal order through a reinterpreted Hartian positivist lens—and understanding both its strengths and limitations in this regard—would be both descriptively and normatively rewarding. Ultimately, such a framework enables us to move beyond the traditional dichotomy of hierarchy and anarchy that has long dominated discussions of international law, and opens the door to a more pluralist, inclusive and flexible understanding of the global legal order.