We may identify three central roles of philosophy in Galen’s work. One, to which he himself gives particular prominence, is the theory of demonstration: this underlies explanations and accounts in the natural world, enables the person trained in it to distinguish secure from merely probable or downright fallacious arguments, and guarantees the reliability of propositions arrived at through its correct application. A second is that Galen’s scientific and medical accounts themselves—of the theory of physical elements, for example, or of causation and change—engage with existing philosophical debates (e.g., between continuum theorists and atomists, or between teleologists and mechanists); and that he here relies strongly on philosophical explanatory categories (e.g., telos, the form–matter distinction). Philosophical technical language and argumentation are also to the fore in specifically medical realms, such as the theory of health or the classification of types of pulse, disease or massage: here too we find a constant drive towards definition and linguistic disambiguation, highly complex schemes of subdivision, and the employment of philosophical vocabulary in their pursuit (e.g., ousia, sumbebēkos, genos, eidos, diaphora).