The epithumetikon or “appetitive soul” is the third and lowest form of soul in Plato’s tripartite schema. It is characterized as fundamentally irrational, with an innate tendency towards excess. This chapter shows that the irrationality of the soul is rooted in its ontological structure. In opposition to thumos, which takes the form of a bodily reaction triggered and directed by reason, the epithumetikon represents the penetration of the flux of the body into the soul. Appetitive desires, pleasures, and pains are directed towards physical objects and arise from changes in bodily states. Moreover, because pain and pleasure are defined respectively as a deviation from and restoration to the body’s natural state, pain has ontological priority over pleasure. Hedonists, who claim to pursue the greatest pleasures in order to lead the happiest lives, are confronted with a dilemma. Either they must choose a life of great pleasure, which necessarily entails great suffering, or they must acknowledge that the life of moderation, not of excess, is the most purely pleasurable.