Although Pseudo-Dionysius was, after Aristotle, the author whom Thomas Aquinas quoted most frequently, surprisingly little attention has been paid to the role of this Neoplatonist thinker in the formation of Aquinas' philosophy. Fran O'Rourke's book is the only available work that investigates the pervasive influence of Pseudo-Dionysius on Aquinas, while at the same time examining the latter's profound originality.

Central themes discussed by O'Rourke include knowledge of the absolute, existence as the first and most universal perfection, the diffusion of creation, the hierarchy of creatures, and their return to God as final end. O'Rourke devotes special attention to the Neoplatonist element in Aquinas' notion of "being" as intensity or degree of perfection. He also considers the relation of being and goodness in light of Aquinas' nuanced reversal of Dionysius' theory of the primacy of the good, and Aquinas' arguments for the transcendental nature of goodness.