The chapter aims at investigating the way Galen constructs his philosophical theories in dialogue with his predecessors, both by adhering and by opposing to their doctrines. For this purpose, it focuses on a certain part of his epistemology, namely his account of sense perception and, in particular, his theory of vision. I argue that Galen’s perceptual theory starts from material he finds in the Platonic dialogues, but revises it significantly either in order to reply to objections raised by Plato’s opponents or in order to rebut unfortunate, at least to his mind, adaptations of the Platonic inheritance. Indeed, in his attempt to defend Plato’s views on sense perception, Galen does not recoil from borrowing whatever seems to him valuable from rival philosophical schools, and it is this enriched reworking of the Platonic theory that he adopts as his own philosophical stance. To fully reconstruct and comprehend Galen’s method in his general theory of sense perception and in his theory of vision, I draw my evidence from what Galen tells us about these topics in his extant works, and especially in the seventh book of his treatise On the Doctrines of Hippocrates and Plato.