… is from page 139 of the 1985 (3rd) edition of the late Ralph Raico’s translation of Ludwig von Mises’s great 1927 book, Liberalism:
Attempts to justify on economic grounds the policy of restricting immigration are therefore doomed from the outset. There cannot be the slightest doubt that immigration barriers diminish the productivity of human labor.
DBx: And, thus, because in a market economy every increase in the productivity of human labor creates more goods and services not only for each of the workers whose productivity rises but also for those persons with whom those workers trade, immigration restrictions diminish the economic welfare not only of would-be immigrants but, also, of all those persons with whom those would-be immigrants would have traded.