In The Law of Peoples, one of his last works, John Rawls sketches a normative theory of international relations amongst decent and liberal nations. He does not discuss immigration and defends his omission by saying that it would not be relevant in an ideal society:
There are numerous causes of immigration. I mention several and suggest that they would disappear in the Society of liberal and decent Peoples. One is the persecution of religious and ethnic minorities, the denial of their human rights. Another is political oppression… Often people are simply fleeing from starvation… Yet famines are often themselves in large part caused by political failures and the absence of decent government. The last cause I mention is population pressure in the home territory, and among its complex of causes is the inequality and subjection of women. Once that inequality and subjection are overcome, and women are granted equal political participation with men and assured education, these problems can be resolved… The problem of immigration is not, then, simply left aside, but is eliminated as a serious problem in a realistic utopia.
I find this passage deeply unsatisfying. Neglecting discussion of immigration by saying that it would not be relevant if the entire world were a decent (or ideal!) society strikes me as manifestly unhelpful. Even if the long-run steady state of ideal societies might result in little concern for immigration policy, immigration between decent and indecent societies would likely be a significant determinant of the transition dynamics to the realistic utopia. Surely liberal societies ought to have concern for immigration policy in this context.
But such thought experiments and omissions might be entirely appropriate in political philosophy. It’s not my field, so I do not know its assumptions and techniques. For the sake of argument, concede everything that Rawls says above and contemplate the “realistic utopia” condition. Rawls is still wrong.
Why? Economic geography! In a world of increasing returns and agglomeration economies, it makes no sense to keep people trapped within historically-produced arbitrary boundaries. It massively decreases human welfare. We want people to be free to gather in densely populated areas and reap the gains of more complex and frequent human interaction. We also want them to move freely to respond to geographically idiosyncratic shocks. People are a resource and borders impose transaction costs that impede allocative efficiency.
For thoughts along these lines, see Lant Pritchett’s “Boom Towns and Ghost Countries.” With labor mobility, there are countries that would become “ghost countries” akin to ghost towns. But due to immigration barriers, there are instead “zombie countries” populated by “human beings, who through no action or fault of their own, are trapped in economically non-viable regions.” Ghost countries are preferable to zombie countries, even in a world governed by “liberal and decent Peoples.”