Concepts such as law, religion or morality may refer both to a practice (or process) and to a doctrine (or product). My thesis is that we should not regard these as separate phenomena, but as two partly incompatible models of the same phenomenon. Law, religion and morality are therefore essentially ambiguous concepts (EAC). An EAC is a concept which refers to a dynamic phenomenon that may only be described and modeled in at least two different ways that are each essentially (...) incomplete and which are partly incompatible with each other. This means that we have to choose between either an incomplete description or an incoherent one. An EAC resembles an essentially contested concept (ECC) as both allow of conflicting conceptions. However, for an ECC the explanation of pluralism lies in the evaluative character of the concept, for an EAC it lies in the dynamic character of the phenomenon. The insight that many of our social concepts are essentially ambiguous may provide a better understanding of social and customary rules and of the debate between legal positivism and its opponents. I reconstruct the positions of three contributors to this debate, Fuller, Hart and Dworkin. An EAC framework offers a better insight into their theories, highlights the deficiencies, and explains why their debates were characterized by serious misunderstandings. (shrink)