This article deals with the problem of the second best in a politically differentiated Europe. Drawing on Kant's category of permissive law of public right, it establishes the missing link between ideal and nonideal theory and then an indirect legitimation theorem. This theorem provides tools to contain damages and unlock practices and dynamics that obstruct reforms and lead to counterproductive outcomes. The theorem, which is equipped with a time and knowledge index, awaits the formation of favourable circumstances of integration. It establishes the conditions under which differentiated integration would be nonregressive. The theorem rules out backsliding solutions, which undermine the European integration process.This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.