A Pure Libertarian Theory of Immigration

"Immigration. The very use of the word is telling: since every immigrant is also an emigrant, why is the first term more frequently used? In the twentieth century, with the exception of countries under socialist regimes, it has typically been harder to move into a particular country than to leave it—but of course, since most of the earth is now under the control of states, leaving a country usually means having to be accepted into another."