Aquinas on definition and essence

Thesis

The thesis presents a systematic investigation into Thomas Aquinas’s metaphysical essentialism by focusing on his interpretation of Aristotle’s Metaphysics Z. The investigation is centered on Aquinas’s interpretation of the ’notion of the ‘logical mode’ that opens Aristotle’s treatment of essence in Z 4. While much of the scholarly tradition has interpreted Aquinas’s modus logicus as either preliminary or dialectical in nature, my work argues that Aquinas method is demonstrative and properly applied to the metaphysical field: it employs logical intentions as valid starting points for metaphysical conclusions.

Chapter 1 provides a conceptual and textual foundation for the modus logicus, arguing against its reduction to dialectics and showing its legitimacy as a metaphysical method.

Chapter 2 examines Aquinas’s treatment of essence in terms of per se predications and real definitions in his Commentary on Z 4, identifying a set of logical and definitional criteria labelled under the Predicative Simplicity Criterion (PSC). These criteria establish the conditions for something X to be the essence of Y. Eventually, the treatment of Aquinas’s Commentary on Z 4 is integrated with a passage from Summa Contra Gentiles II, 58, where Aquinas argues in modus logicus from the reality of per se₁ predications to the reality of the ontological unity of the essence. I call it the Predicative Simplicity Argument (PSA).

Chapter 3 investigates the problem of the hylomorphic composition of sensible essences in Aquinas’s interpretation of Z 10-11, showing how Aquinas applies his modus logicus to prove the reality of prime matter and the necessarily enmattered character of a sensible essence. This leads to a metaphysical account of essence as a unified and simple principle, despite its composite nature. Aquinas’s perspective on Z 10-11 is also critically compared with some contemporary Aristotelian interpreters.

Chapter 4 analyzes Aquinas’s Commentary on Z 12 and his solution to the problem of the unity of a genus+differentia definition. I show how the logical problem of the unity of the definition can be solved by showing its fundamental relation with the unity of the hylomorphic essence. I call Aquinas’s model of the unity of the definition UTM (Unitarian Transformative Model). As a matter of fact, Aquinas’s elaboration has a striking similarity with the discussions in contemporary ethics and philosophy of mind on additive vs transformative accounts of human rationality.

Authors: Banfi, F

• DOI: 10.5287/ora-4rdak2g7x
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: medieval philosophy; Aristotle; Aquinas; essentialism; Book Zeta; metaphysics; logic