This chapter discusses the place and the foundations of the theory of sources in the European tradition of international law, with an emphasis on the work of Pierre-Marie Dupuy. It starts by recalling how the theory of sources grew into the linchpin of the cognition and identification of international law in the mainstream European tradition of international law. The chapter then shows the extent to which Pierre-Marie Dupuy, despite his critical stance towards the mainstream theory of sources, has stood out among his peers by constantly seeking to unearth the theoretical foundations of the sources of international law. Finally, it formulates a few epistemological remarks on the extent to which the vindication of the theory of sources reflects some underlying choices on what it possibly means to be an international legal scholar in the European tradition of international law.