The paper examines the notion of medium in Aristotle's epistemological and ontological thought. The paper holds that Aristotle’s philosophy contains two different conceptions of mediums: the first is the medium of “image” as a representation of the entity and the world in early ancient Greek philosophy, which is clearly expressed in Democritus and Plato; the second is the elemental medium of air, water, fire, and earth as the constituent entities of the world. The former was incorporated into the theory of “form” by Aristotle and constituted the core connotation of the “representationality” of existence in Western philosophy; the latter was rediscovered only after the rise of mediology. Not only did Aristotle put forward the concept of elemental media and the media of form, but also his binary distinction between form and material, potential and reality contains a profound understanding of the medium of knowing and being, which has important implications for the current media studies.