In his 29th Treatise, "On Vision", Plotinus addresses the difficulty of knowing the actual function of the medium ("metaxu") separating the sensible object from the eye. The first four chapters of the treatise are dedicated to this question: in order to specify the role of the medium in visual perception, Plotinus provisionally admits its necessity and, in the first chapter, draws up a list of the various theoretical models conceivable on the basis of this working hypothesis. In order to get a better view of the stages and issues of this dense and elliptical argument, barely touched upon in studies of this issue, I analyze it in light of some earlier texts about the modalities of sensation as such and the problem, more generally, of transmitting an affection through a medium. These questions were addressed before Plotinus within various conceptual frameworks and according to the views of various authors (Plato, Aristotle and his Peripatetic followers, Chrysippus, Plutarch of Chaeronea, the Elder Pliny and Galen), and they were reworked by Alexander of Aphrodisias on a specific theme related to cosmology. I hope to show that the new model of transmission that he proposed allowed Plotinus to elaborate his own theory of vision, on the basis of a process of disaffection of the perceptive medium that was already initiated before him.