Allow me to translate that from Justinian to "Person with a soul."
(Just playin, you know I love ya!)
Since nations have limitations, be it geographic, demographic, or economic, that prevent them from taking in every person in the world, they have to limit the amount of immigration. In addition, obvious security concerns mean governments would wish to prevent completely open borders. So a nation must choose between people, rather than accept everyone.
So the question becomes one of how to choose who enters the country.
Since any individual becomes a shared burden (social services, use of public goods) and contribution (taxes, work) to a society, it's in the nation's best interests to prefer taking in individuals who give the greatest contribution to the society, to mitigate their burden. From there, it becomes simple mathematics: If one person has a potential to accrue $100,000 per year or fill a highly demanded niche (i.e., a scientist, teacher, doctor), such a person should be preferred over an unskilled person since the former has a greater probability to cover their costs. By accepting individuals who are less likely to cover their social costs, the nation doesn't reap the benefits of the immigration.
Even accepting unskilled, yet highly sought-after, immigrants probably isn't enough. Every new entrant into a labor market increases the supply of labor in a field, reducing the value of that labor, along with all other workers in the field. Unskilled work uniquely has the quality of being simple for people to enter in case a shortage does occur. For example, if every plumber except for one guy disappeared suddenly, new plumbers could, and would, enter the field easily, because the work isn't that complicated.
This is untrue of fields such as medicine, making immigrants in such fields highly sought after. Therefore, we should court them above unskilled labor, because it's simply efficient to do so.
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