Galen exemplifies and expounds this view at length in his refutation of Chrysippus in PHP II and III; such relationships between physical states and mental ones are also explored in a number of medical texts too (for a full range of such texts, and further discussion, see Singer 2017). Galen makes interestingly precise attempts at identifying physical correlates for a range of psychological or emotional states, in terms of the precise actions and states of heart or blood, e.g. fear involves a motion of the blood away from the surface of the skin, anger the opposite motion, both erotic excitement and anxiety cause significant disturbance to the pulse. Yet the texts in question are unclear on the precise nature of the causal (or identity) relationship. Relevant here too are the medical discussions in which it is clearly implied that certain types of physical state—e.g., excess of melancholic humours—are causative, if not constitutive, of certain mental states. At points Galen even suggests that the soul may actually be the physical substrate, in particular the blood and pneuma within the body (Symp. Caus. II.5, 191) or more specifically the pneuma within the brain (PHP VII.3, 443–5); he seems however ultimately to reject such a notion, preferring the formulation that the pneuma is the soul’s “first instrument for all sensations of the animal and for its voluntary motions” (ibid.).