The present work seeks to overcome an impasse in Catholic moral theology by retrieving the crucial relationship between the acting subject and the object of the act as described in Thomas Aquinas's analysis of human action. The contentious debate between proportionalists and their critics following the Second Vatican Council—it is here argued—has been fueled on both sides by an inadequate understanding of Aquinas's nuanced account of the relationship between the subject and the object in moral action. Drawing from John Paul II's encyclical Veritatis splendor, this thesis suggests that understanding article I-II.18.6 of the Summa Theologica as central in Thomas's analysis of action offers a way to think about subjectivity and objectivity as mutually constitutive approaches to moral action. Subject and object, like the interior act of the will and the external act, depend on one another for their identity, and together constitute the composite whole, as soul and body together constitute the human person.

The need for a retrieval of Aquinas's original insight is explored both in connection the influence of nominalism on the Catholic moral tradition as well as debates concerning the proper relationship of nature and grace. In both instances, one detects a tendency to over-emphasize object, law, obligation, and externality to such an extent the acting subject virtually disappears from consideration. The unfortunate fallout of this intellectual history is highlighted in a critical analysis of the proportionalist debates. It is argued that neither the proportionalists nor their critics had a sufficient account of the acting subject as an inherently social unity of body and soul whose agency is formed within the context of particular communities with particular histories. Accordingly, this thesis argues that a full retrieval of Aquinas's original insights requires attending to the work of Herbert McCabe, O.P., whose account of the human person as a linguistic animal aids us in breaking through the dualistic habits of thought that make it so difficult to read Aquinas on his own terms.