A study of substantial change in the writings of St Thomas Aquinas

This dissertation examines substantial change as explained by St Thomas Aquinas in a number of his works. It provides a systematic exposition, explanation and defence of his account of substantial change, arguing that it is not only satisfactory but also in accord with a sound philosophy of nature as well as being metaphysically consistent.

The central aim of the dissertation is to explain how substantial changes are said to occur, that is, to explain the process of substantial change. This process involves a transition from potency to act, which constitutes the essence of change. The explanation of the process of substantial change is said to be a hylomorphic explanation, in that it involves the postulation of two per se principles of nature, namely prime matter as the potential principle and substantial form as the actuating principle, and one per accidens principle, namely privation.

This dissertation deals with its topic in five chapters. The first chapter deals with some considerations preliminary to the investigation of substantial change. It considers what is meant by substance, the argument that there are many different substances, and the evidence of substantial change. There is then examined three possible explanations of substantial change, namely annihilation/creation, transubstantiation and a substratum theory. St Thomas’s explanation is identified as a substratum theory, and more particularly as a hylomorphic version of a substratum theory. According to this substratum theory, substantial change involves one substantial form replacing another in the underlying substratum of prime matter. The central aim of the dissertation is to explain how the prime matter undergoes the transition from potentially possessing a substantial form to actually possessing it. The second chapter examines the three principles of change, namely matter, form and privation, beginning with accidental change and then arguing by way of analogy to substantial change. At the end of the chapter, five difficulties or objections are raised, which are then answered in subsequent chapters. The fifth difficulty is in fact the principal problem addressed in the dissertation, namely how to explain the origin of the new substantial form in the prime matter.

The third and fourth chapters examine the process of substantial change and in particular respond to the principal problem of the dissertation. Three possible explanations for the origin of substantial forms are examined, namely that the form was actual but latent in the prime matter, that it was created by an external agent or that it was educed from the potency of prime matter. St Thomas argues for the third explanation of eduction, from the Latin ex ducere, meaning ‘to bring out of.’ The fourth chapter examines in detail the process of eduction by which a new substantial form is produced. In particular the role of dispositions in prime matter is examined. Prime matter is said to be indirectly disposed by means of changes in the accidents inhering directly in the composite supposit, i.e., the individual substance. The fifth and final chapter considers the objections raised at the end of chapter two in light of some modern authors and replies are given to these objections. It is concluded that St Thomas’s account is sufficiently robust to provide a philosophical explanation of substantial change based upon metaphysical principles.