The simultaneous unity and plurality of law was a central topic for Hans Kelsen. He conceived of unity and plurality in accordance of the so-called black box model. For him, global law consisted of territorially differentiated national legal orders as well as international law addressing the relationships between these legal orders (or the states personifying them). Legal orders were self-contained normative entities which treated each other as black boxes. Their internal unity was created by their formal, hierarchical, structure. In our post-Kelsenian age, both unity and plurality, as well as their interrelationship, have to be rethought. Unity can no longer be built on the formal characteristics of law. In Europe, for instance, not only does EU law itself ignore the formal features of a Kelsenian hierarchical Stufenbau; through the fundamental principles of direct effect and supremacy, it has also shaken the internal hierarchy of Member State legal orders. Unity is still possible but must be constructed from substance and conceived of in dynamic terms. In addition, unity must be related to plurality. Legal orders are not self-contained normative entities, but they are linked to one another through a variety of relations which can be brought together under the notion of interlegality. At issue is not only such express interlegality as characterizes the relationship between EU law and national law but also interlegality which works at the sub-surface, legal cultural levels of law. All in all, interlegality is a major reason for the failure of the Kelsenian black box model. Plurality can no longer be understood as simple legal diversity; we have to take the theoretical step from diversity to pluralism. The rise of transnational law is an important, but not the only, backdrop to the problematization of the traditional way of conceiving of the unity and plurality of law.
This project forms part of the STIAS long-term project Boundaries and Legal Authority in a Global context within the Crossing Borders research theme.