Consistency in international law: how to make sense of a decentralised and expansive rule-based world

Unlike domestic law, where rule-making is the job of legislative houses, rule-making in international law is decentralised. No central authority or unified government creates international legal rules. Instead, its subjects build international law by signing treaties, practising customs, and recognising general principles. A consequence of decentralisation is that most of the time, international legal rules have no clear priority relationships between them. The scenario grew even more complicated after 1945 with the expansion of international law. We are undergoing a period of accelerated growth in the number of rules of international law; at the same time, its thematic scope is diversifying and its institutions are proliferating. This scenario leaves the subjects of international law in a tight spot, as it is common for them to face a mass of applicable rules that, if applied, would lead to incompatible outcomes. That is, these subjects often face conflicts. Conflicts leave subjects uncertain about their legal positions, which, in turn, hinders their ability to plan and decide what to do.

Against this background, this PhD dissertation’s research question asks how subjects can extract a consistent set of legal positions from international law’s ruleset despite the fact that international law is decentralised and ever-expanding. This dissertation explains that there is a significant difference between statements and rules, which leads to different concepts of consistency. There is the consistency of statement sets (S-consistency) and the consistency of rulesets (R-consistency). A statement set is S-consistent if all statements in that set can be true at the same time. If they cannot, that set is S-inconsistent. A ruleset is R-consistent when it cannot lead to conflicts. If it can lead to conflicts, it is R-inconsistent. With these two kinds of consistency in place, this dissertation advances a two-part thesis: more rules do not always lead to a ruleset’s R-inconsistency; even if a ruleset is R-inconsistent, we can still extract an S-consistent statement set on what outcomes obtain as subjects’ legal positions. This thesis leads to two corollaries: international law’s decentralised expansion does not necessarily lead to its R-inconsistency; even if international law’s ruleset is R-inconsistent, subjects can make sense of it by extracting an S-consistent statement set on their legal positions. Subjects can extract S-consistent statement sets from R-inconsistent rulesets by reasoning with and about rules to figure out what rules apply and what outcomes obtain and, so, reveal their legal positions.

This dissertation is divided into six chapters. Chapter 1 introduces the research. Then, chapter 2 explains international law’s decentralised expansion. Chapter 3 clarifies that the complexity underlying international law’s decentralised expansion is one of uncertainty concerning the legal positions of subjects. Chapter 4 develops a logical framework for reasoning with and about rules to help subjects figure out their legal positions. Chapter 5 draws on earlier chapters to supply some applications for the theory developed in this text. Finally, chapter 6 concludes this study by summarising its main findings.