This essay seeks to reflect on how the conscious political effort to overcome the divisiveness caused by Member States' idiosyncracies has matured into the constitutional recognition of Member State identities as essential to the European project. Central findings concern the particular twist from 'national identity' to 'constitutional identity' in the Lisbon Treaty. This can be considered a codification of the case law of some of the Member State constitutional courts. This implies that, whereas the Lisbon Treaty failed to incorporate a 'supremacy clause' on the priority of EU law over conflicting Member State law, it has indeed formulated a legally binding rule on the priority of certain Member State constitutional law over EU law. The more precise contours of this priority have already become the object of controversial ECJ case law. This essay explains why this European controversy must necessarily remain open-ended, based as it is on tolerance, the acceptance of otherness and trust.