Aquinas on Parts, Wholes, and Aristotelian Anti-Reductionism

Abstract

The paper discusses Aquinas’s Aristotelian claim that substances are not aggregates of either their ordinary parts or their metaphysical constituents. The analysis is conducted through a survey of Aquinas’s mereological theory against the background of standard contemporary mereology. It is argued in particular that Aquinas rejects three fundamental principles of standard mereology, i.e. Monism, Extensionalism and Universalism. The paper focuses in particular on Aquinas’s rejection of Monism and endorsement of Pluralism, and argues that Aquinas distinguishes three fundamental kinds of whole: mixtures, relational wholes and hylomorphic composites (substances). The distinguishing features of hylomorphic composites with respect to the other kinds of whole are illustrated by considering Aquinas’s thoughts about actuality, potentiality and existence.