Aquinas’s Ontology of the Material World: Change, Hylomorphism, and Material Objects by Jeffrey E. Brower (review)

What is the ultimate structure of the material world? Brower’s monograph provides a well-argued reconstruction of Aquinas’s Aristotelian answer to this question: the fundamental contents of the material world are prime matter, substantial and accidental forms, substances, and accidental unities.

Brower’s aim is twofold: presenting Aquinas’s ontology of the material world, and also emphasizing its relevance—as a mixed ontology and a peculiar two-tier structured substratum theory, combining elements of both thin and thick particularism—to current metaphysical debates. These two perspectives intermingle in the five sections of the book. In part 1, Brower provides a clear roadmap and lucidly condenses the key elements of his interpretation of Aquinas. In parts 2–4, Aquinas’s ontological account is gradually spelled out, starting from his analysis of change in physics. In part 5, this account is adjusted in view of some theological and anthropological refinements.

According to Aquinas, any change involves two hylomorphic compounds, made out of one portion of enduring matter and two incompatible forms inhering successively in it. As emphasized in part 2, however, Aquinas in not only a realist, but also a functionalist as far as his hylomorphism is concerned. Accordingly, matter, form, and compound designate three metaphysical roles played by three really distinct things, rather than three specific categories of beings. Hence matter, form, and compound can be described in terms of potentiality and actuality. For instance, matter is anything potential, capable of receiving different forms’ actualization over time, and playing the role of subject either of mere endurance (prime matter in substantial change) or of endurance and characterization (substratum in accidental change)—which, in a realist framework, entails the existence of both prime matter and “kooky objects.”

In part 3, Aquinas’s metaphysics of prime matter is systematized. Brower’s realistic reading is not original, although his intention to unfold the ontological type to which prime matter belongs is pioneering and stimulating. Neither universal nor individual, in his view Aquinas’s prime matter is in actuality in some very broad sense: not through itself, but via inherence, as a gunky, atomless stuff. Re-identifiable over time, compoundable, and divisible, it possesses some special type of numerical oneness in all things, that is, by composition: “there exists a single sum or portion of prime matter composed of all the distinct, smaller portions existing in the world” (117).

Although Aquinas commits himself to some universalism about matter, he rejects it when material composition is at stake: there is no hylomorphic compound without some form inhering in some matter. Moreover, Aquinas identifies prime matter with corporeal matter. Hence, as Brower shows in part 4, he admits of two subsets of hylomorphic compounds: material and immaterial ones. While to the former subset belong both substances and accidental unities possessing prime matter, to the latter belong only accidental unities, whose substrata are angels. Brower then investigates the relationship between material compounds and material objects. Resorting to the notion of “numerical sameness without identity,” he argues that a material object is numerically the same as the hylomorphic compounds with which it is co-located.

This book is addressed to both medievalists and contemporary philosophers. It risks perhaps disappointing the former, if anyone: close textual analyses could be less infrequent, translations less interpretative. Moreover, although Brower’s historical remarks are always precise and insightful, he is too selective. For instance, some references to the debates on the unicity of substantial form and to the notion of common matter would have been welcome.

Brower’s choice of taking change as his starting point is innovative and well-grounded. In so doing, however, he tends to reduce any change to generation and corruption—which sounds odd within an Aristotelian physical framework. For Aquinas acknowledges that local motion is, to some extent, presupposed by any other kind of change. This difficulty surfaces when Brower focuses on celestial bodies and claims that...