Although Aristotle’s thought became a byword in the early modern period for a rigid and inflexible system impervious to empirical evidence, it seems to have been quite otherwise in the practice of Aristotle’s own school. In reconstructing the psychological theory of the third head of the Lyceum, Strato of Lampsacus, I argue that we can best understand his departures from Aristotelian doctrine by looking to new medical theories in the Hellenistic period, which had implications for our philosophical understanding of the soul’s functioning. Aristotle, who was interested in the natural processes underlying even the most abstract aspects of human functioning, had lampooned the notion that the soul might exist apart from the body: his psychology posited an intimate connection between form and function, and was informed by empirical studies of living organisms. Strato was known even in antiquity both for a revisionist attitude and his focus on natural philosophy. It would thus be unsurprising for him to be interested in new medical research.