Date of Award

Spring 2023

Document Type


Degree Name

Doctor of Philosophy (PhD)



First Advisor

Twetten, David

Second Advisor

Taylor, Richard

Third Advisor

Goldin, Owen


There is a distinctively Avicennian way of understanding the categories to be found in the works of Thomas Aquinas that vindicates Aquinas’s early argument for the distinction between being and essence. Two of the most important and influential Aquinas scholars in the twentieth century recognized the roots of this Avicennian way in Aquinas, but neither Etienne Gilson and Cornelio Fabro made good on their insights. In this dissertation, I trace this Avicennian way through its sources in the Greek commentators and demonstrate how it provides the necessary insight into the structure and nature of the categories that render Aquinas’s Genus Argument intelligible. When one studies the history of the phrase “being per se” as a (quasi) definition of substance, one encounters a tradition of reading the categories after Aristotle that regards them as categorizing essences to which “being per se” or “being not in a subject” is like a property, concomitant, or “completer” of substance. Avicenna, as both a recipient and participant in this tradition of late antique commentary, recognizes the need to clarify in what sense of “being” the phrases “being per se” or “being not in a subject” can—and, more importantly, cannot—designate a property or completer of substance. Thanks to Avicenna, Aquinas resumes this tradition. For Aquinas, the ten genera categorize essences that are distinguished according to their diverse modes of being—modes that are, in one sense, proper to them and, in another sense, owed to them as something external. Only when we understand this background, can we understand Thomas’s “Genus Argument” not as a mere logical argument, subject to a fallacious inference from logical to ontological considerations. Just as we must ascribe to Avicenna an argument from the (quasi) definable essence of the categories to an extramental distinction between essence and “being in act” (wujūd bi-fi‘l), so Aquinas’s Genus Argument is a form of Avicenna’s “Categories Argument” from the structure of the ten genera or categories of reality.

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