  Telesio combined the medical theory of spirit with a basically Stoic notion, that of the hegemonikon, according to which the spirit in the brain is responsible for all the states and operations traditionally ascribed to the tripartite soul: “the animal … is governed by one substance residing in the brain” (Quod animal universum ch. XXV; Var. lib. p. 254). Whereas in Quod animal universum he went on to explain the affects in terms of physiology, in De rerum natura he added a theory of sense perception and a theory of knowledge on physical grounds.