The principle, omne agens agit sibi simile, "every agent causes something similar to itself," is fundamental to Scholastic metaphysics, and especially natural theology. In fact, it is only upon its vasis that inferences can be made from creaturely characteristics to the nature of the Creator. However, omne agens agit sibi simile, is taken for granted even by an author such as Saint Thomas Aquinas, who never feels any need to justify its validity, in spite of the fact that "there is hardly a phrase which occurs more often in Saint Thomas," as Etienne Gilson remarked.
Tracing the historical roots of omne agens agit sibi simile is an indispensable first step in trying to explain the import of this principle in Scholastic Thought. The first part of the book is devoted to this task. it argues that the mediaeval metaphysics of causal similarity is rooted in a conception of the cosmos which goes back to the Presocratics, and according to which being is essentially circular, or self-reflexive. This conception was further elaborated by Plato, Aristotle, the Neoplatonists, and their mediaeval successors. The second part examines omne agens agit sibi simile in Thomistic metaphysics. Without neglecting Aquinas's sources, it attempts to elucidate the structure of his thought in the light of contemporary philosophical questions. It is stressed, for instance, that in Aquinas's thought, causality involves a process of 'concealing revelation" of the cause in and through its effect―an idea which was later to become a central element in Heidegger's philosophy.