My research covers the following areas:

Until 1998, I was mainly a historian of ideas. This period was concluded with the publication of my PhD-thesis (The Disintegration of Natural Law Theory: Aquinas to Finnis, Brill, Leiden, 1998), in which I tried to refute the assumption that law should be founded in universal and eternal principles which can be deduced from (human) nature. I argued in that book that nearly all the attempts in that direction had ended in failure. After that, I only incidentally turned to history.

Probably in reaction to the failure of natural law to ground law in extra-legal morality, I began to get interested in the thought that the criteria for the evaluation of law should be found in the nature of law itself as a formal, procedural, public and explicit system. In such a perspective formal procedures are valuable, also and even if they fail to contribute to a certain desired result.

From 2005 onwards, when I started working at the Academy for Legislation in The Hague, I began to develop an interest in types of rules which seem to lack entirely this kind of procedural value: the many frame- directives and goal-prescriptions that require the norm-adressee to achieve a certain predetermined result. Not the road but the destination is the main concern of many forms of alternative (principle-based) regulation. Gradually I discovered that this type of rules is accompanied by a particular style of enforcement and specific types of sanctions. This discovery led to further research in several directions.

One such direction is the debate on the methodology of legal scholarship. Initially regarding this debate as a mere side-affair, it gradually dawned upon me that contemporary criticism of legal scholarship is linked intimately with the style of regulation that was the subject of my research. Diminished interest in the importance of procedures and rules is reflected in a diminished tolerance for a proper methodology for legal scholarship.

From 2019 I became more and more interested in the evolution of normative orders in general and of international law in particular. Overlapping legal orders as well as rivalling normative orders, their successes and failures can hardly be understood on the basis of the assumptions that are dominant in the theoretical reflections on national legal systems. I am currently trying to develop an evolutionary theory that gives a better insight into the development of normative orders.