Throughout St. Thomas Aquinas's works one finds several general metaphysical principles frequently employed. Among these are "everything received is received according to the mode of the receiver," "every agent produces something similar to itself," and "the maximum in any genus is the cause of all the others in that genus." St. Thomas uses this last principle in passages ranging from his early works, such as his Commentary on the Sentences and his De ente et essentia, to his mature works, such as his Summa theologiae, Summa contra Gentiles, and Quaestiones disputatae . He employs this principle in contexts as varied as his discussion of the charity we owe to God and creatures, his treatment of the primacy of contradiction among the kinds of opposition, and his "fourth way," the fourth of five arguments he proposes in Summa theologiae I, q. 2, a. 3 to establish God's existence. Moreover, the terms of this principle have different meanings across these texts. Thomas applies it to what are genera properly speaking as well as to the "genera" of the transcendentals, and, moreover, to cases regarding different kinds of causality.

Some commentators stress the Platonic character of this principle, but he consistently cites Aristotle's Metaphysics II as his source. This text, however, seems to suggest the converse of the principle as Thomas usually cites it---that the cause of all the other things in a genus is also the maximum in that genus. Perhaps the principle is convertible, but how may one see this? Some scholars suggest that Thomas holds the principle to be self-evident. There are, however, texts where he provides some argumentation for it. I offer here a careful textual study of this principle in its various forms and applications with a special emphasis on Thomas's philosophical justifications.