The chapter explores the interaction of reason with perceptual experience and empirical procedures in Galen. It aims to answer two main questions: (a) how does Galen's account cope with the problem of the inexpressibility of sensory experience in words? (b) what input does logos – in the sense both of prior theoretical conceptions, and of verbal accounts – have in the organization and training, and/or the actual content, of sensory experience? The investigation is carried out on the basis of a number of substantial texts which have previously received very little scholarly attention, and in particular examines in conjunction relevant passages from the treatises on the pulse and from that on simple drugs. I identify and discuss Galen's distinctive and original theoretical arguments in these areas, as well as the relevance of his discussions to medical training and clinical practice.