During the fifth and fourth centuries bce, a number of Greek doctors attempted to base the art of healing on the first principles of all things in general. These “cosmological doctors” included such thinkers as Eryximachus, Philistion, Petron, the unnamed opponents of On Ancient Medicine, and the authors of On the Nature of the Human Being, On Breaths, On Flesh, and On Regimen. Previous studies have approached these thinkers under the rubric of medicine's interactions with “philosophy.” This book, by contrast, will approach them from a medical point of view, arguing that the best way to understand these systems is to view them as responses to preexisting modes of medical thinking.