Substantive and Procedural Rules in International Adjudication: Exploring their Interaction in Intervention before the International Court of Justice

In this chapter we will follow the thread of the separation of substantive and procedural rules in international adjudication and its importance in international law, using as our example intervention before the International Court of Justice (ICJ). As succinctly put by Judge Weeramantry “intervention affords an example par excellence of the celebrated observation that substantive law is often secreted in the interstices of procedure. The subject is therefore one of special importance, not merely in the sphere of procedure but in the sphere of substantive law as well.”1 We begin our analysis by briefly sketching the origins of this separation, to demonstrate its importance, while noting that boundaries are not only blurred but also permeable. We then turn to examining the history and practice of intervention before the ICJ. More specifically, a typology of interactions shows how procedure can uphold and reflect the values carried by substantive rules and how substantive rules can in turn shape the interpretation of procedural rules. Our goal is to draw an impressionistic picture of the role of intervention through a different and largely under-explored angle of the interaction between substance and procedure. I.