The Problem of Thomistic Parts

Thomas Aquinas embraces a controversial claim about the way in which parts of a substance depend on the substance’s substantial form. The substantial form is responsible for the identity/nature of the parts of the substance such a form constitutes. Aquinas’s controversial claim can be roughly put as the view that things are members of their kind in virtue of their substantial form.
The aim of this paper will be to defend Aquinas’s claim that every time the xs come to compose a y, those xs have to undergo a change in kind membership. 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 thoroughgoing naturalistic methodology that resolves such problems by appeal to empirical considerations.