While the preceding two chapters focused on the physiological domains whose motions take place ‘by nature’, that is, involuntarily, this chapter looks at the activities of the physiological system responsible for the motion ‘by will’. Galen depends on Hellenistic anatomists, especially Herophilus, for much of what he knows about the nervous system, but this chapter looks at both inherited knowledge and polemic interaction. In a rare case of disagreement, Galen criticizes Herophilus regarding the claims about the inherent sensitivity of the nerve tissue. The fact that Galen does not accept Herophilus’ experiments and maintains that nerves only receive capacity from the brain shapes his understanding of this physiological domain. The activities of the nervous system encompass not only voluntary motion but also sense perception and pain, and this chapter argues that each of them has distinctive implications for the unity of the living body as a whole.