Opinio juris: Test, filter, ideal or map?

Despite widespread criticism by both practitioners and theorists, the so-called two-elements theory in which state practice should be accompanied by opinio juris is still persistently maintained as the cornerstone of customary international law. This chapter seeks to clarify this undiminished support by investigating the various possible functions of opinio juris as additional element. Does it serve to recognise normativity? Or to identify legal validity? After having criticised the conceptual problems of these various assigned functions, an alternative function is proposed: opinio juris is not a statement of the law but a claim about the law. Such a claim represents the rules of practices at a different meta-level: 'these are the rules.' But this is not a merely descriptive task. Opinio juris can better be regarded as a map, which represents the law by selecting and highlighting those aspects that are deemed important. It describes and prescribes at the same time. Opinio juris is therefore not circular because it is not an expression of established legal validity but is a claim for legal validity. Without such representations, legal discourse is not possible, which explains why opinio juris is usually seen as an indispensable element.