Abstract The difference between ancient Greek medicine and ancient Greek philosophy has often been seen by scholars in terms of two targets of expertise: the body and the soul. In this paper, I argue that we can better understand the boundaries between medicine and philosophy in antiquity by focusing instead on the difference between causes and motivations (or causes and desires). The reason is this. It is not the case that the writers of the Hippocratic Corpus are uninterested in the soul (psychē). They are, however, reluctant to address their therapies to expressions of the patient’s own agency, despite tacitly acknowledging such agency as a causal force that cannot be reduced to the automatic behavior of the body. I go on to show how thinkers like Plato and Democritus zero in on the problem of perverted desires as part of a strategy of establishing a new domain of therapy, a domain that comes to be classified as the therapy of the soul.