This article is a contribution to the field of study that Jacques Maritain once described as “metaphysical Axiomatics.” I discuss Aquinas’s use of the metaphysical principle “omne agens agit sibi simile,” focusing on perhaps the most manifest instance of this principle, namely, univocal generation. It is well known that Aquinas holds what could be called a “static” or “formal” view of likeness between God and creatures: creatures are like God because they share in certain exemplar perfections that preexist in God. My focus instead is on an efficient likeness to God, which reflects a foundational truth about reality for Aquinas: all creatures produce something like themselves through their operations, in imitation of God, who does so on a more fundamental level. My discussion will also clarify Aquinas’s derivation of the principle of similitude from a prior metaphysical principle, “every agent acts insofar as it is in act.”

Catholic Tradition | Contemporary Philosophy | History of Philosophy | Philosophy and Religion