It is important to understand the anatomical-medical, as opposed to narrowly philosophical, background here. Galen’s theory is heavily influenced by the work of the Hellenistic medical authors Herophilus and Erasistratus (fourth–third century BCE). This crucially included the discovery of the nervous system, as well as some detailed knowledge and theorization of the anatomy and functions of the brain (all, of course, quite unknown to Plato, with whose theory Galen attempts to harmonize them); see von Staden 2000. Galen’s conception of brain, heart and liver as “sources” (archai) of the three Platonic parts of the soul is inextricably linked with his anatomical understanding of the three sets of channels (nerves, arteries, veins) through which they operate.