The episcopate is established by positive indefeasible institution; and there are reasons why it should be: yet who shall pretend that presbyterian government, and ordination by an assembly of presbyters, is, absolutely speaking, an intrinsic absurdity? Or that it could not have been instituted by Christ, had He so willed? Here, as repeatedly in this work, St Thomas's necesse est is no argument of antecedent necessity, but of mere congruentia post factum.

Of God and His Creatures: 4.76