Those who advocate open borders, or at least a huge liberalization of EU immigration policy, have an ally in the influential Australian philosopher Peter Singer, who has argued that all people in the West must make painful financial sacrifices of most of their disposable income to help the world's poor:
Peter Singer has argued in Practical Ethics (1993) that you are morally deficient if you eat meat, or if you fail to give a good bit of your income – 5% if you earn more than $100,000, and at least 10% of income over $150,000 – to help the world’s most destitute. It’s actually worse than that. If you take Singer’s arguments seriously, you should be giving nearly everything you have to charity. (Singer himself doesn’t go that far, giving away only 20% of his income. Nobody’s perfect.)
...Singer’s basic argument is simple, relying on two main principles. Somewhat paraphrased, these principles are, first, maximize pleasure and minimize suffering; and second, all pleasure or suffering counts equally. (Because of Singer’s particular interests, the bit about minimizing suffering plays a larger role than the bit about maximizing pleasure.) You can question how to apply these principles in particular situations, but for Singer there are no principles more fundamental.
One immediate consequence of Singer’s principles is that animal suffering should weigh as heavily in your decisions as human suffering: that’s part of what he means by ‘all suffering counts equally.’ Animals may not suffer as much as humans, but whatever their suffering, it’s as significant as an equal amount of human suffering.
Another consequence of treating everyone’s – sorry, every organism’s – suffering the same, is that your suffering doesn’t count more than anyone else’s. Since there are so many people in the world who suffer more than you, it follows that you should give a substantial part of your wealth to alleviate that suffering...
To convince you that you should give more of your wealth to alleviate suffering, Singer uses a persuasive analogy. Suppose you see a child drowning in a pool. You can rescue the child at no danger to yourself, but at the cost of ruining your new suit (PE, p.229). Clearly, you are morally obliged to wade in, suit be damned. But, says Singer, if you are a moderately well-off citizen of a first world nation, donating 10% of your income to CARE or Oxfam will similarly relieve much suffering, with only a modest impingement on your lifestyle (p.222). As with the drowning child, you can’t just walk by. You have to grab your chequebook and wade on in.
Using similar principles, Singer concludes that you must be a vegetarian, 'that you shouldn’t give your own children extraordinary advantages' and that we should encourage very old people to kill themselves -- perhaps even kill them ourselves -- so that we can spend the €200,000 it costs to prolong Grandma's life for 4 months to immunize 1000 poor children. Some of these things Peter Singer believes, others are thought experiments designed to foster discussion (and boy, do they ever). He cheerfully admits people will never do most of these things, but they should.
I don't know whether Peter Singer has endorsed open borders between Europe and Africa, but I can imagine he would. And therein lies the problem: almost no Westerners have ever lived up to Peter Singer's idea of completely moral conduct, and they never will. And most of them are OK with that, don't like being scolded as immoral, and think that they are nevertheless decent people. The counter-arguments to Singer are manifold, starting with Hume, who declares it to be perfectly normal and understandable that we care more about those closest to us than those far distant. (The linked article sets out all the critiques). Catholic social teaching holds the same view. And virtually every human alive does too, especially if we judge them by their actions, not their words. Plus, you can't develop a workable ethical system without context-based compromises:
In real societies, and especially in large-scale modern societies, there are a profusion of competing ethical principles. In speaking of ‘competing principles’, I don’t just mean that different people have different principles (although they do), but that there are many principles, in competition with each other, guiding any single person’s actions. All those principles can’t all be true all the time. We harmonize them, to the extent we can, by adjusting the contexts in which we see them as applicable.
If ethical rules arise out of the rough and tumble of harmonizing our own interests, including our social impulses, with the interests of others, and with the contingencies thrown up by an infinitely-various natural world, then the rules we come up with are likely to be partial rules for the here and now, not universal rules which will work in all situations, especially those far from our experience; and there are likely to be a large number of rules, each applicable in a small if ill-defined context. For even the most basic ethical rule, there will be contexts where it clearly applies, contexts where it clearly doesn’t apply, and a large grey area in which there can be indecision and controversy. ‘Thou shalt not kill’, for example, is an unimpeachable moral principle, but we can still argue about its range of legitimate application. Self-defense? Just wars? Abortion? Euthanasia? Animals? Vegetables? Around the sizeable edges, there is plenty of room for dispute. It’s not a criticism of a rule to admit that it’s not always clear where it applies.