Arnheim's media theory is introduced amidst the uncertain context of the contemporary post-medium condition. After decades of suspicion of both medium and media systems, some robust theory of medium is needed. Nevertheless, those process-based accounts of medium introduced recently with the turn to "materialism" and "ontology," and, influenced by Deleuze, do not directly address an affirmative account of medium, instead substituting partial systems intended to account for the determinacy of media in a half-hearted way. Against caricatures of traditional ontological theories as "essentialistic," or defining fixed and eternal natures, it is argued that they are sufficiently rich to stake out a variety of commitments and to deal with the persistent aspects of media without erecting a rigid and reductive system. Indeed, the conflation of the possession of powers with their actualization confuses the description of a medium and the norm of its use.