Stephen L. Brock

Aquinas describes transubstantiation as a “conversion” of one substance into another. Yet he denies any common substrate underlying the succession of substances. Germain Grisez finds this unintelligible. The article's thesis is that Aquinas saw and resolved the basic issue contained in Grisez's objection. The key text stresses a “nature of being” common to the two substances. This nature, it is argued, is univocal. As such it constitutes a continuous object of signification that is both necessary and sufficient for the sacramental action of the words of the consecration. That action can only be understood as a supernatural kind of conversion.

The general aim of this paper is simply to draw attention to a certain theme in St Thomas' psychology of human action, one not often treated at much length in discussions of Aquinas on action. This theme is his notion of usus , "use", as a stage or component in the accomplishment of a complete human act. I shall begin by indicating some possible reasons for the general disregard of the theme, and shall then briefly note some rather striking affirmations by Aquinas concerning use, affirmations which suggest that it is probably more interesting, both for him and for us, than it might at first seem. The core of the paper focuses upon a set of problems about the coherence of Aquinas' notion of use. There my interlocutor will be Professor Alan Donagan, who has been one of the few writers on action-theory in Aquinas to dwell on this subject. The proposed resolution of these problems will, I think, make it at least plausible to assign to the act of use a rather central role in the account of Aquinas' mature way of conceiving the human will, as a faculty distinct from the intellect and as primarily a faculty of agency. The paper closes with a nod to one of the more significant features of Aquinas' moral philosophy that I think his doctrine of use helps us to keep in mind