Introduction

Central to the study of the way in which law constrains the external exercise and effect of the public power of states is an understanding of the relevant aspects of the relationship between public international law and municipal law. After all, it is the very province of public international law to regulate the public power of states as between themselves. The extent and manner in which its rules are given effect within the municipal legal system are therefore vital to the operation of foreign relations law.

Yet the essential starting point is that public international law and municipal law are two separate legal systems. Each has its own internal secondary rules of recognition, which determine, for the purpose of that system, the validity and interpretation of legal norms. Such rules include rules about the extent to which the norms of other systems have currency or application within the system. ‘Each legal system has its own rules of reception: that is what it is to be a legal system.’ International law has such rules to determine its priority over national law in establishing the responsibility of states on the international plane, but also to decide how it must proceed when the issue before the international tribunal is one properly governed by municipal law. Municipal law has a highly developed set of rules that decide when foreign law is to be applied: that is the function of private international law.