Thomas Aquinas embraces a controversial claim about the way in which parts of a substance depend on the substance’s substantial form. On his metaphysics, a ‘substantial form’ is not merely a relation among already existing things, in virtue of which (for example) the arrangement or configuration of those things would count as a substance. The substantial form is rather responsible for the identity or nature of the parts of the substance such a form constitutes. Aquinas’ controversial claim can be roughly put as the view that things are members of their kind in virtue of their substantial form. To put it simply, Aquinas’ claim results in the implication that, every time the xs come to compose a y, those xs have to undergo a change in kind membership.
This has been called the “homonymy principle,” and it follows from Aquinas’ view of substantial forms, and specifically from the position that substantial forms inform prime matter, rather than substance-parts. The aim of this paper will be to defend that the Thomistic claim that substantial forms account for the determinate actuality of every part of a substance is plausible and coherent. After defending the Thomistic account, I propose that approaching problems of material composition as a Thomist has a significant, oft-overlooked advantage of involving a thorough-going naturalistic methodology that resolves such problems by appeal to empirical considerations.