Central to the understanding of clinical practice, too, is that we are here dealing with a system of symptoms or signs; and inference from signs cannot give one the same kind of secure knowledge that is available for propositions arrived at through demonstration. Galen here engages in what was already a complex and sophisticated area of discussion, drawing on both Hippocratic texts (see the discussion of signs in his commentary on the Hippocratic Prognosticon) and debates in Hellenistic philosophy and medicine (Allen 2001, Hankinson 2008a, van der Eijk 2008). As we have seen Galen, unlike the Empiricists, believes that the observation of bodily signs—a complex range, for the ancient doctor—enables the expert to make inferences about the underlying bodily state and causal conditions. The term endeixis here refers to the “indication” that the observed body gives, both of what has caused its present state, and of the appropriate treatment.