This chapter argues that the cosmological doctors arose as the result of a much wider realignment in Classical Greek medicine. As some doctors grew increasingly concerned about the many variables that can change from one case to the next, they rejected older forms of diagnostic handbooks in favor of new methods for organizing medical knowledge. We see this anxiety over individual differences not only in the works of the cosmological doctors but also in texts such as On Regimen in Acute Diseases, Prognostic, Airs Waters Places, and the seven books of Epidemics. In all of these texts, medical inquiry is defined, quite generally, as a search for commonalities. Doctors in this period were gathering together multiple accounts, noting the similarities and differences between those accounts, and isolating high-level generalizations that can unite and govern them all. Although the cosmological doctors took their search for commonalities farther than some of their contemporaries might have been willing to follow, they nevertheless responded to the same pressures that transformed nearly all the medical literature that survives from this period.