My research interests cover a wide range, both in themes and in disciplines. I studied law and moral philosophy, and have a strong interest in sociology and social theory. In my research I try to combine these three disciplines. There are four major themes in my research.

Legal scholarship and ethics as hermeneutic, normative, and empirical interdisciplines
Throughout my career, a recurring theme in my teaching and in my research has been the methodology of ethical and legal research. Initially, I studied methods in ethics and developed constructivist and pragmatist interpretations of the reflective equilibrium method (Reflective Equilibrium 1998). Following my move to Tilburg Law School, I focused on methodological issues of law as an interdisciplinary discipline. Three recent sabbaticals in Sydney and London enabled me to explore this theme in depth and it is now the core theme of my research. The starting point is the methodological deficit in legal doctrinal research: namely, the fact that the academic discipline of law lacks generally accepted and rigorous research methods. My research question is how to justify evaluative judgments and normative recommendations for law reform in legal research. A subquestion is how we can incorporate other disciplines and their findings into legal doctrinal research; on this theme I have co-edited a double special issue of the Erasmus Law Review with Sanne Taekema (2015).

The dynamic relation between law and morality
This has been a central theme of my research since I wrote my master’s thesis on civil disobedience (published in 1986). With Pieter Ippel, I edited a book on the relation between law and morality in the field of biomedicine, De Siamese tweeling (1994). Two more recent perspectives in this research are the themes of dynamics and pluralism. How can we understand dynamics and pluralism in law and morality, and how can law and morality deal with them? How may the academic disciplines of law and ethics respond to the phenomena of global legal pluralism and legal and moral change? This was the theme of my Tilburg inaugural lecture Dynamic Law (2001) and my article ‘Dynamic Ethics’ (2003). In this context I studied interactionist views on law such as those of Lon L. Fuller (Rediscovering Fuller 1999, with Willem Witteveen) and Philip Selznick, as well as communicative and interactive approaches of legislation. This research line has recently resulted in my book The Dynamics of Law and Morality. A Pluralist Account of Legal Interactionism (Ashgate hc 2014, Routledge pb 2016).

Democracy, the rule of law and (cultural and religious) diversity
In my doctoral dissertation A democratic point of view (1991) a central theme was how a democracy should deal with cultural and religious differences and with differences in gender and sexual orientation. Since then, this has been a recurrent theme in my research. More recent publications are the book On religion, morality and politics. A liberal alternative (2005), the ‘preadvies’ for the Dutch Lawyers Association ‘Cultural diversity and the democratic state with a rule of law’ (2008), my Rotterdam inaugural lecture The ideal of the neutral state (2009), and a number of articles on state neutrality and the separation of state and church (e.g., ‘What is neutrality?’ 2014, with Roland Pierik). I have also contributed to the Dutch public debate on cultural and religious diversity through many public lectures and publications for a broader public.

Ideals in law, morality, and politics
Ideals are a neglected category in morality, law and politics (and in academic theories of those fields.) They play an important role in the dynamics of law, morality, and politics, in pluralism and debates and in the understanding of the interaction between law and ethics. Between 1996 and 2002, I led a research group of more than twenty researchers on this theme, financed by a NWO-PIONIER grant. The concluding book was The Importance of Ideals (2004, co-edited with Sanne Taekema).