Although Pseudo-Dionysius was, after Aristotle, the author whom Thomas Aquinas quoted most frequently, little attention has been paid to the role of this Neoplatonist thinker in the formation of Aquinas's 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.