This chapter proposes solutions to some longstanding problems surrounding the anthropogony of On Flesh. First, it shows how the author’s three main principles of the hot, the cold, and the wet reflect widely attested beliefs about the effects of heat and cold on bodily fluids. After that, it argues that the author’s two supplementary principles of the “fatty” and the “glutinous” are derived from a traditional dichotomy between bile and phlegm. The upshot of these observations is that the author of On Flesh uses the microcosm of the body as a tool for understanding the macrocosm of the universe. For this author, the natural world is primarily a reflection of the body (not the other way around), and it is specifically medical knowledge that gives him insight into the cosmos. Just as Eryximachus claims to have acquired his awareness of the universal power of eros “from medicine, our art” (ἐκ τῆς ἰατρικῆς, τῆς ἡμετέρας τέχνης, Pl. Smp. 186a), so the other cosmological doctors viewed medicine as a privileged starting point for contemplating the universe as a whole.