Abstract

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.