Addressing these design problems depends on your view of a just society. For this I appreciate Rawls's Veil of Ignorance: imagine creating a just society, but not knowing what your future place in that society will be. You could end up as a rich person on the fortunate side of the border, or a poor person on the unfortunate side.
If the possibility of not winning the borders lottery makes you queasy, then you know that morality wise borders shouldn't be a thing anymore.
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