The Thin Justice of International Law constitutes an ambitious attempt to demonstrate and convince both international lawyers and philosophers that international law is supported by a moral foundation which contributes to the development and maintenance of a just world order. Steven Ratner hopes that this will incentivize more robust interdisciplinary engagement with international law and philosophical ethics in order to advance global justice through international law.

In Part I, Ratner presents his analytical framework. Firstly, by examining current debates on global justice—including, from the international legal perspective, both Bruno Simma's concept of community interests and the New Haven School's conceptualization of law—he determines that an approach comprehensively integrating international law and philosophical ethics is largely lacking (Chapter 1). In Chapter 2, Ratner provides the groundwork for such an approach, to enable the assessment and promotion of international law's contribution to a just world order. He addresses both the conceptual issues pertaining to the identification of the standard of justice and the method by which the just nature of international legal rules will be assessed. Equipped with an international lawyer's insights into the political and institutional constraints of the realm in which the law operates, Ratner adopts a non-ideal standard of justice, which is ‘thin’ because it is claimed to represent a moral minimum. This is comprised of two core principles—or ‘pillars’ as he prefers to call them—which emerge from a process of internal discovery and justification which he undertakes. The first is the advancement of interstate and intrastate peace, and the second is the non-interference with basic human rights. The way in which these pillars are operationalized in assessing the just nature of international legal rules is subsequently explained. The rule of law—which, for Ratner, includes considerations of fairness—and effectiveness, as evidenced by a substantial degree of compliance, are also deemed relevant to some extent to this assessment (Chapter 3).

In Parts II–IV, Ratner appraises international law's core components against the two pillars of justice. A thematic structure underlies the assessment of a broad range of rules, including the ‘core norms on statehood’ (Part II), those relating to ‘territorially-based protections of human rights’ (Part III) and the ‘core norms of the global economy’ (Part IV). Ratner's international legal expertise enables him to provide a succinct and clear summary of the relevant rules, despite debates and uncertainty regarding their content. Each section commences with a synopsis of the rules, which are then evaluated against the pillars of thin justice. The previously determined components of international law are taken into account in the assessment of the rules pertaining to the membership and decision-making rules of international organizations, as the pillars adopted are often, but not always, irrelevant in this context. Given that the standard of justice is derived from the observation of international legal rules, the conclusion that most norms are in fact just is somewhat unsurprising. Nevertheless, Ratner's approach does not equate to an apology for the status quo. Some norms, such as the blanket ban on humanitarian intervention as well as aspects of the law on immunities, are found to be unjust.

In Part V, Ratner acknowledges that not all core rules can be assessed against this standard of justice as their relationship with, and effect on, these pillars remains indeterminate, while the additional considerations are also unable to assist in this context. This limitation of his standard of justice relates to international humanitarian, criminal and environmental law. Nevertheless, Ratner provides various reasons to support the conclusion that the main rules of these sub-areas of law should be respected (Chapter 12). Ultimately, a ‘thicker’, yet still non-ideal, theory of justice is proposed which essentially reverses the pillars, making human rights the prime consideration, while also expanding the scope of relevant rights considered and requiring that the rules not only refrain from interfering with such rights, but rather promote them. The process by which legal reform can be pursued to secure the compatibility of international legal rules with ‘thick’ justice, while ensuring that the thin justice attained thus far is not jeopardized, is also briefly outlined (Chapter 13).

The Thin Justice of International Law provides a unique perspective on the ethical underpinnings of the international legal order and the ability of international law to contribute to a more just world order. The methodology adopted in ascertaining the standard of justice as well as the content of this norm, can be subjected to various criticisms, for example the exclusion of economic welfare as a third pillar, or of certain rights as part of the second pillar. Yet, the standard adopted largely corresponds to current principles, rules and practices of States, thereby introducing a practically relevant standard that reflects much discourse on international law's normative agenda. Furthermore, the double (observational and justificatory) screenings utilized for the identification and adoption of the two pillars shield the standard against criticisms were either approach used in isolation. Moreover, although positivist readers are likely to disagree with some of Ratner's conclusions on the substance of international legal rules, it is mostly clear when a contested stance is adopted (as is the case, for example, in the context of secession and unilateral humanitarian intervention). Nevertheless, such a stance becomes somewhat problematic where the just nature of interrelated rules are assessed on the basis of a prior, contested stance on a related rule.

Irrespective of the validity of Ratner's conclusions on the content of the law or the extent to which particular rules are just, and insofar as the book's principal aim is to stimulate discussions from both international lawyers and philosophers on the international legal order's potential to contribute to a just world, The Thin Justice of International Law is undoubtedly a source from which related interdisciplinary debates can emerge. It is noteworthy that Ratner recognizes several shortfalls and uncertainties intrinsic to his assessment, including, for example, by presupposing the necessary existence of States, while also making an explicit choice not to assess the fairness of the process by which international legal rules arise, despite accepting that political and historical factors are relevant in this regard. However, he tactfully considers both ethical and legal critiques and alternatives to his conclusions. Ultimately, as a practical matter, it remains unknown whether an argument that international law can and does enhance global justice, however persuasive, will either affect States’ compliance with such rules or persuade them to seek reforms to ensure that the thin—and ultimately the ‘thicker’—justice is attained.