Some of the extreme opacity of the idea of custom seems to be owed to two considerations. One is that custom modifies the actors who observe it; a well-established custom is not merely a constraint external to a person's character, but has a tendency, with time, to constitute an aspect of his being. Another is that customary arrangements have typically involved an interaction between two overlapping groups of people: the community within which given customs are observed and a much smaller group perhaps most neutrally described as the custodians of such arrangements, a group who are in practice charged with the articulation of the customs, and therefore with their application and development. A link between these two considerations is that the very function of the custodians is likely to demand internalisation of methods, moral attitudes, and intellectual habits appropriate to the social norms that they articulate. At all events, these generalities were true of the pre-modern common lawyers. From the perspective of a book on custom, the interest of pre-Benthamite common law thinking lies in the tension it exhibited between its accounts of what the lawyers did – the patterns of thought and behaviour that might be said to constitute a trained professional – and its attempts to identify their esoteric practice with principles engendered by the wider population.