Abstract: 'Iustitia est constans et perpetua voluntas ius suum cuique tribuens.' One of the ways to make 'iustitia' (justice) substantial is by using principles. Studying the role of principles in cases governed by various fields of law (e.g., private law and European Union law) makes clear that principles can play a role in two ways. The first is the role recognized in modern private law: principles functioning within a system with detailed rules and bringing as an extra to that system an interpreting, supplementing or derogating function. The second way can be discovered in European Union Law: the generative function of principles used out of necessity to build up a young system and to generate and develop rules in more detail. Cases governed by more layers of law, for example Angonese and Mangold, show that principles indeed may play a role different from the one they play in regular private law cases and that the landscape for the private law lawyer has been reshaped slightly and will be reshaped even more intensely. That being a fact, the question is how a private law lawyer is supposed to define his reaction to these developments, these facts? It is a question that is hardly stressed in the broad debate about multi layered law.
The aims of a private law contribution to European Union law in my view are (a) that private law gets integrated in European Union law by adjusting to the aims and principles of European Union law; (b) that it takes part in the process of development, refinement and sophistication of European Union law; (c) that it remains a smoothly functioning part of multi layered law; (d) that private law preserves its ability to give each one his due in a more or less predictable way; (e) that private law for that reason develops its function in supporting the effectiveness of European Union law and (f) that actors in European Union law (e.g., ECJ, legislator) be more aware of the impact of European Union law on private law and for that reason (g) take this impact into due consideration in judgments and provisions. To achieve these aims a two-way traffic between European Union law and private law must be established. The question is: how can private law lawyers, judges and scholars become involved? Which approaches should be chosen in case of confrontation of a field of law, private law, containing refined rules, with a field of law, European Union Law, containing less detailed rules and more concrete policies that do not specifically aim at private law relationships? In other words, which approaches should be taken when confronting a system that has developed into utmost refinement with a system that has started to be built up only a few decades ago. How can these parts of the law be bridged?European Review of Private Law