How Do You Know if an Action Is Good or Not? Not Easily.

The short answer is, it’s very difficult.

Catholics today wrestle with the same ethical questions they did centuries ago.

In Good & Evil Actions: A Journey through Saint Thomas Aquinas, Steven J. Jenson examines Aquinas’ view on one such topic: how to discern if an action is good or evil.

It’s a debate even Thomists are divided on.

In the book’s foreword, Dr. Ralph McInery contemplates Aquinas’ posture toward such sensitive subjects.


By Dr. Ralph McInery

It is inescapably true that we should do good and avoid evil, but how are we to know the difference between them? Whenever St. Thomas faces this question, he quotes the Psalmist in the Vulgate (Ps. 4, 6): Quis ostendit nobis bona? The answer is: signatum est super nos lumen vultus tui, domine. Who will show us what is good? The light of the countenance is sealed upon us, o Lord. That is, by a participation in the divine wisdom, men have a natural capacity to discern good and evil.

One becomes a good person by performing good deeds; a primary task of moral philosophy, accordingly, is getting clear on the nature of the human act.

Human acts are acts that humans perform, of course, and it is important to begin with such resounding truisms.

But difficulties arise and one learns to appreciate Thomas Aquinas’ distinction between the human act and the act of a man. Not every activity truly ascribed to a man counts as a human act, the mark of the latter being that it is deliberate and voluntary. Inhaling and exhaling, dreaming, and the like thus do not count as human acts. One must know what he is doing and freely do it in order for the act to be human.

Thus one is drawn into a consideration of the constituents of the human act, the contributions of reason on the one hand and of will on the other. There is the inner act as well as the external action, and moral appraisal of the one or the other poses different though related questions. The constituents of the complete human act enable us to appraise imperfect actions that do not reach their term and yet form part of our moral history.

So it is that students of St. Thomas have long since learned the complexity that awaits them in following initial leads of fundamental certainty. Along the way, a host of problems arise, more or less central, and little schools are formed. Each of the constituents of the human act raises issues of its own.

Notably, the notion of the intention of the agent calls for lengthy reflection. In short, an ample menu of difficulties confronts the student who wants both to be guided by St. Thomas and to learn from the efforts of others.


For a penetrating study on this topic, read Steven J. Jensen’s Good & Evil Actions: A Journey with Thomas Aquinas. It’s included in the CUA Thomas Aquinas Moral Theology Collection (8 vols.)—now on Pre-Pub for 27% off.

Written by

Verbum, part of the Logos family, empowers Catholics to study Scripture and explore Church tradition.

View all articles
Written by Verbum