The definition of the European Union (EU) from a positivist standpoint depends on the definition of its relationship with the Member States. This definition in turn depends on the way the conflicting interpretations of the principle of primacy of EU law by European and national courts are reconciled. Based on an analysis of European and national case-law, three different positions exist on the matter. According to the European Court of Justice, which has adopted a monist approach with primacy of EU law, the principle of primacy of EU law is absolute, which is reflected in terms of validity in the subordination of Member States to the European Union, which is thus a European state or a monist internal legal system, characterized by its single ground of validity. According to the national courts of Member States, which have adopted a monist approach with primacy of national law, the principle of primacy of EU law is relative only, and the validity of EU norms in the national arena is defined by higher national norms. Whatever the definition of EU law, whether it is defined as an international legal system or a sui generis one, the EU is considered as a sub-system of the national legal system, and thus as an internal monist legal system. According to the national courts of Member States, which have adopted a dualist approach, the principle of primacy of EU law means simply that EU norms take precedence over national norms in terms of implementation, which is determined by national rules. Whatever the definition of EU law, whether it is defined as an international legal system or a sui generis one, the EU and the national legal system are defined as separate albeit coordinated legal systems. This diversity of positions based on the legal system chosen as the frame of reference, reflects the common feature shared by all legal systems: their sovereignty. It is at the basis of their relationship, which is a pluralist one. This does not mean however that EU law is an international legal system, as this characterisation would not reflect the integration taking place between Member states and the EU. A complex system of direct interactions between European and national organs, correlative to the integrated system of EU/Member States material competences, has given to this relationship a tri-dimensional shape. The EU stricto sensu and the Member States are at the origin of a third common legal system: the EU largo sensu which forms a pluralist internal legal system, encompassing both the EU stricto sensu and the Member States, and which is characterised by a plurality of grounds of validity: those of the EU stricto sensu and those of the Member States. The mystery of the legal nature of the European Union is thus solved.