In this article, we discuss a particular form of interdisciplinary legal research. We focus on a discipline that may be fruitfully combined with doctrinal research, namely philosophy. The aim of this article is to give an account of the methods of philosophy that are most relevant and useful for doctrinal legal scholars. Our focus is therefore mostly on legal philosophy and the philosophical subdisciplines closely related to it, such as political philosophy and ethics. We characterize legal philosophy in three complementary ways: as an activity, as insights, and as theories. We then discuss three methods of legal philosophy: argumentation analysis and construction, author analysis and reflective equilibrium. In the practice of research these three methods are usually combined, as we will show with various examples.