In On the Nature of the Human Being, Polybus claims that the humors, like everything else, are composed of four substances: the hot, the cold, the dry, and the wet. He also discusses humoral flux – the idea that a single humor, after gathering in one part of the body, can create a wide range of effects by flowing to different parts. This chapter argues that Polybus’s thinking about cosmological principles is closely tied to his interest in humoral flux. Both concepts are motivated by an interest in tracing diseases to their “source” (arche), and both ultimately establish a two-pronged approach to treatment, wherein doctors are expected to target both the proximate cause of a disease (i.e., a concentrated humor) and its remote cause (i.e., the factor(s) that initially caused that humor to separate out). This chapter further argues that an interest in individual differences does not preclude a coexisting interest in cosmological principles. Instead, Polybus presents his cosmological principles as a solution to the problem of individual variation, identifying the shared “nature” (phusis) of human beings as a high-level commonality that transcends individual differences.