An ideal called ‘the rule of law’ has won praise since ancient times from a diverse band of enthusiasts ranging from Aristotle and John Locke to the former UN Secretary-General, Kofi Annan. This apparent consensus, one that impressively bridges temporal, cultural and ideological divides, partly reflects the obvious fact that the phrase ‘the rule of law’ means different things to different people.1 One can, without linguistic impropriety, use this expression to refer to a number of significant though distinct ideas, or indeed to no clear idea whatsoever. To make progress, we must disentangle some of these ideas before trying to identify and elaborate the most compelling or fruitful version(s) of the rule of law.

Inquiry into the nature of law as a form of social ordering, including the values it ought to serve, threads a long and complex path through the great classics of Western philosophy. Plato’s Republic and Laws, Aristotle’s Nicomachean Ethics and Politics, Cicero’s De Officiis, St Thomas Aquinas’ Summa Theologiae, Thomas Hobbes’ Leviathan, Immanuel Kant’s The Metaphysics of Morals, and G.W.F. Hegel’s The Philosophy of Right, all engage with questions that form part of such an inquiry. But even though many of the chapters in this volume display an acute sensitivity to this history, with some authors explicitly situating themselves in a specific tradition of philosophical thought, they all share a determinedly contemporary focus. The contributors to this volume were not invited to provide overviews of the history of ideas, but rather to identify what they judge to be the key questions for us today, and the most compelling lines of approach to those questions, in relation to topics such as the aims and limits of law; what makes for sound legal adjudication; the scope of justice and the place of human rights; the concepts, institutional structures, and values that unify individual departments of law such as crime, contract, and tort; and the prospects for transnational legal order. This contemporary focus is neither a mere accident nor an instance of sheer parochialism.

Immanuel Kant notoriously declared that it was a “scandal of philosophy” that it had not yet furnished us with a convincing proof of the existence of an external world. International lawyers have their equivalent occupational scandal: the failure to achieve clarity or consensus on the nature of customary international law. Custom, after all, is arguably the most fundamental source of international law, at least insofar as treaty law is itself embedded within a customary framework. This framework includes various principles bearing on the interpretation of treaties and arguably also the grundnorm of treaty law, pacta sunt servanda. Indeed, the international lawyer's scandal goes deeper. All of us, philosophers or not, standardly proceed on the basis that a world external to our senses exists. By contrast, assertions about customary international law are largely confined to international lawyers, although their being taken seriously occasionally has real practical consequences for others.

It is not enough to respond to this state of affairs with a knee-jerk pragmatism: the shop-worn thesis that customary international law works well enough “in practice” and so requires no explication “in theory.” After all, this simply presupposes that we already know what customary international law is, and merely shifts attention to whether it “works.” In any case, it is doubtful that anything can satisfactorily “work” in a discursive and legitimacy-claiming practice if its very nature remains stubbornly opaque or conceptually problematic. Equally, we should not be put off by the skeptical dogma that all of our moral-political ideas are infected with contradictions at their very core, so that the search for an explanation that makes good sense of them is doomed from the outset. Even the alluring consolations of intellectual resignation need to be earned by argument rather than mere fiat.

In this chapter, by drawing on, clarifying, and extending previous work, I try to sketch the argument that the pragmatists and skeptics take to be either unnecessary or impossible. I offer a moral judgment-based account (MJA) of customary international law, one that challenges the orthodox idea that there is a deep connection between custom and consent, and I mobilize the ensuing account in relation to human rights norms in particular.

Contemporary international law – including its institutional agents and mouthpieces, such as the United Nations, World Trade Organization, and International Court of Justice – asserts the “legitimacy” of its claim to govern an increasingly broad, diverse, and consequential array of activities within the global domain. However, this very domain manifests high levels of divergence in the cultural norms; patterns of economic and legal organization; religious, moral, and political creeds; and so on that are prevalent within and across its constituent states. In light of these deep and ramifying differences, how can international legal norms and institutions avoid reflecting a partial or “parochial” perspective, with the result that their “legitimacy” is seriously imperiled? This chapter explores this rather vague, but prima facie genuinely disquieting, line of thought by considering two distinct, but readily confused, ways in which “parochialism” might undermine the legitimacy of international law. The first sees the challenge as arising from meta-ethical skepticism about ethical values in general; the second, by contrast, treats it as the upshot of the ethical doctrine of pluralism. The conclusion toward which this chapter steers is that skepticism, although more radically subversive in its implications for the legitimacy of international law, is not a promising way of articulating the anxiety about parochialism. By contrast, the less-frequently examined pluralist challenge poses a significant constraint on international law's legitimacy, albeit one that still leaves plenty of room for the legitimacy of a robust international legal order.

What Is Legitimacy?

What is “legitimacy”? According to a recent book-length study, the language of “legitimacy” has become “dominant and recurrent” in contemporary international relations, upstaging the language of legality, justice, and efficiency. Perhaps somewhat cynically, one might suppose that its dominance at the level of international political rhetoric is not unconnected with its indeterminacy of sense, or at least with the lack of widespread agreement among international law practitioners on an univocal concept of legitimacy. Even if it is true that, as the author of the aforementioned study goes on to suggest, the concept remains “widely ignored in the academic discipline of international relations,” the same cannot be said of contemporary legal and political philosophy, in which legitimacy has been the focus of sustained discussion by a number of influential writers. Without yet immersing ourselves in the intricacies of this work, there are two philosophical commonplaces about legitimacy that should orient our discussion from the outset.

Hope without fear?

Customary international law (CIL) excites both hope and fear among international lawyers. It excites hope among partisans of the idea that international law is an important means for advancing the cause of global justice; indeed, they would go further and insist that international law's legitimacy – the very justifiability of its claim to set normative standards for its subjects – turns on whether it gives adequate expression to the demands of global justice. I use the label ‘global justice’ in a broad sense, to encompass moral values and principles, including but not limited to the minimalist values of peaceful co-existence and co-operation, that should be respected by all states whether in their internal affairs (transnational justice) or in their relations with other states (international justice). The requirements of global justice include norms establishing human rights, authorizing humanitarian intervention and assistance, enjoining environmental protection, and so on. But how can morally attractive standards of this sort, which are regularly flouted throughout the world today, acquire the status of international laws that are opposable against all states?

Reliance upon treaties is prey to well-known difficulties: not all states have signed up or are likely to sign up to norms of the relevant type. Moreover, many states that have formally agreed to such norms have done so subject to eviscerating reservations.