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Human history is the history of migration and the most sophisticated civilizations arose where human traffic was heaviest. The Ancient Near East, the Indian sub-continent, China, the Americas, Europe – all had constant influxes of migrants bringing new ideas and change. And in ancient Greece the Delphic priests regarded the right of unfettered movement as one of four freedoms distinguishing liberty from slavery. Because they did not feel responsible for newcomers, rulers often saw them as an asset rather than a liability. They would add to a region’s wealth, contribute to taxes and serve in local armies.

R5: No, no, no. Even many pro-open borders groups acknowledge that the lack of a passport and visa system reflected the limited capacity of the central government as much as it did inherent tolerance. While Rome was generally among the more tolerant ancient civilizations, Athens for instance jealously guarded citizenship and Rome (even setting aside the slavery thing) had requirements for obtaining the vote as well as, yup, walls.. Sparta was infamous in the ancient world for its deportations, to the point where there was a word for it - xenelasia - in Spartan Greek. Even indigenous people were regularly deported - the Jews, repeatedly, were evicted from the Levant and exile was an accepted criminal punishment in Athens. So no, the ancient world was not an open borders utopia...arbitrary and capricious deportation of people with no other homeland was accepted.

People moved constantly all over the world – be they Vikings, Crusaders or Chinese emigrants. Large scale restrictions, however, were imposed with the introduction of serfdom in Europe under the Roman Empire during the third and fourth centuries AD. Initially, controls were lax but under the Roman Emperor Constantine (AD 309–37) serfs were forbidden to leave their work place and had to accept whatever conditions their lords imposed. The Romans even introduced the first ‘passport’ – a document requesting safe passage for the bearer. By mediaeval times a large part of Europe’s population was bound in place and traded like chattels. Movement was considered inimical to order and the possibility for human migration was restricted mainly to wars.

R5: The sentence about serfdom was debunked in the first paragraph; restrictions on both migration and citizenship predated Constantine, and Vikings and Crusaders were in no way peaceful immigrants (both also occurred after Constantine "shut the borders" too). According to even a very brief Google search, something resembling a passport is found even in the Old Testament of the Bible. And lastly, that sentence at the end that implies migration decreased after Constantine is kind of iffy; that period was in fact a heyday of migration within Europe, the Levant, and North Africa.