Abstract
Can the existence of a social practice justify practical authority? A medieval debate between hierocrats and caesaropapists may help to illuminate this question. Focusing mainly on Marsilius of Padua, with reference to John of Paris, this article suggests that caesaropapists can be read as developing a 'practice conception' of the structure and scope of ecclesiastical authority. Because it brings the conflict over authority to a new battleground, the practice conception supplies caesaropapists with a source of dialectical leverage over hierocratic doctrine. The paper explores the strengths and limitations of this methodological approach and links it to debates in contemporary political thought.