We must, first of all, ask what a barbarian is or could have been in our sources. There were different kinds of barbarians, based upon the Roman and Greek ethnographic tradition and view of geography. Greek writers defined identities of human societies in the known world and bequeathed ethnonyms. Since the sixth and fifth centuries B.C.E., these categories were most often little more than learned constructions. Greek ethnographers like Hekataios, Herodotus and Eratosthenes of Cyrene categorized the world north of the Alps as a western Keltike (Κελτική) and an eastern Scythike (Σκυθική) with the river Tanais (Don) as its frontier. Only Celts and Scyths were known as the two ethne (ἔθνη) living in the northern part of the inhabited world. Ethne was understood as greater groups of peoples.