Lycurgus (27.3-27.4)

This was the reason why he did not permit them to live abroad at their pleasure and wander in strange lands, assuming foreign habits and imitating the lives of peoples who were without training and lived under different forms of government. Nay more, he actually drove away from the city the multitudes which streamed in there for no useful purpose, not because he feared they might become imitators of his form of government and learn useful lessons in virtue, as Thucydides says,[*] but rather that they might not become in any wise teachers of evil. For along with strange people, strange doctrines must come in;

and novel doctrines bring novel decisions, from which there must arise many feelings and resolutions which destroy the harmony of the existing political order. Therefore he thought it more necessary to keep bad manners and customs from invading and filling the city than it was to keep out infectious diseases.