This is a book about the ways in which various intellectuals in the post-classical Mediterranean imagined the human community as a unified, homogenous whole composed of a diversity of parts. More specifically, it explores how authors of the second century CE adopted and adapted a particular ethnic and cultural discourse that had been elaborated by late fifth- and fourth-century BCE Athenian intellectuals. At the center of this book is a series of contests over the meaning of lineage and descent and the extent to which the political community is or ought to be coterminous with what we might call a biologically homogenous collectivity. The study suggests that early imperial intellectuals found in late classical and early Hellenistic thought a way of accommodating the claims of both ethnicity and culture in a single discourse of communal identity. The idea of the unity of humankind evolved in the fifth and fourth centuries as a response to and an engine for the creation of a rapidly shrinking and increasingly integrated oikoumene . The increased presence of outsiders in the classical city-state as well as the creation of sources of authority that lay outside of the polis destabilized the idea of the polis as a kin group (natio). Beginning in the early fourth century and gaining great momentum in the wake of Alexander's conquest of the East, traditional dichotomies such as Greek and barbarian lost much of their explanatory power. In the second-century CE, by contrast, the empire of the Romans imposed a political space that was imagined by many to be coterminous with the oikoumene itself. One of the central claims of this study is that the forms of cosmopolitan and ecumenical thought that emerged in both moments did so as responses to the idea that the natio - the kin group - is (or ought to be) the basis for any human collectivity.