Below is an excerpt from Bernard Lonergan’s incredible book Grace and Freedom, discussing Thomas Aquinas’s views on causation and how they relate to Aristotle’s views on the topic. Except for the term “actio” I’ve replaced Latin phrases with their English translations in square brackets.

Causation is the common feature of both operation and cooperation; its nature is of fundamental importance in this inquiry. But if St Thomas certainly disagreed with Hume, who held causation to be purely subjective, it is less clear what object he considered to constitute the objective reference of the proposition “A causes B.” Was causation for him something in between A and B? Or was it simply the relation of dependence of B on A? Or was it some entity added to A as actually causing? Let us take each of these three views in turn.

As to the first view, that causation is in between cause and effect, St Thomas constantly and explicitly denied it in the case of divine activity. Avicennist biology had distinguished between a [a moving power commanding (something)] and a [a motive power effecting (something)], and St Albert had drawn a parallel distinction between the [divine created power] and a [divine uncreated power]. But St Thomas, while he used the biological opinion at least in his commentary on the Sentences, always asserted that God was his own virtue, operated without any mediating virtue, indeed operated [by the immediacy of power]. The matter is less clear with regard to causation by creatures. Even in later works there is a variety of expressions which appear to imply something in between agent and recipient. Still, it should seem that these are but modes of expression or of conception; for what is in between, if it is something, must be either substance or accident; but causation as such can hardly be another substance; and if it were an accident, it would have to be either the miracle of an accident without a subject, or else, what St Thomas denied, an accident in transit from one subject to another.

On the second view, causation is simply the relation of dependence in the effect with respect to the cause. This is the Aristotelian position presented in the Physics and explained by St Thomas as follows. First of all, this analysis prescinded from the case of the mover being moved accidentally; for instance, a terrestrial body acts through contact and cannot touch without being touched; but this does not prove that the cause as cause undergoes change but only that the terrestrial body as cause does so. In the second place, it was argued that the emergence of a motion or change involved the actuation of both the active potency of the cause and the passive potency of the effect. In the third, place the thesis was stated: one and the same act actuates both potencies, and this act is the motion produced in the object moved. Fourthly, there came the ground of this position: if causation, actio, were an entity inherent in the cause, then, since it is a motion, it would follow either that “[every moving thing is moved],” or else that motion inheres in a subject without the subject being moved; but the latter is contradictory, and the former would preclude the idea of an immovable mover; therefore, causation is not inherent in the cause but in the effect. Finally, the objective difference between action and passion was explained: both are really identical with the motion of the recipient; they differ notionally, for action is this motion as from the cause, [movement of this as from this], while passion is the same motion as inhering in the effect, [movement of this as in this].

It would seem that St Thomas accepted this Aristotelian analysis as true and did not merely study it as a detached and indifferent commentator. Not only did he repeat the same exposition in commenting the parallel passage in the Metaphysics, while in the De anima he argued that sound and hearing, instances of action and passion, must be one and the same reality, else every mover would be moved; but in works that are entirely his own the same view at least occasionally turns up. In the Summa theologiae the definition of actual grace appeals to the third book of the Physics for the doctrine that “[an act of a mover is a movement in the thing moved]”; the analysis of the idea of creation was based upon the Aristotelian identification of action and passion with motion; and the fact that this identification involved no confusion of action with passion was adduced to solve the object against the Blessed Trinity, namely, that since the divine Persons were identical with the divine substance they must be identical with one another. Still, this is not the whole story. In his commentary on the Sentences St Thomas brushed aside the notion that action and passion were on and the same reality, while in the parallel passage in the Summa theologiae a solution is found that does not compromise the authority of Aristotle. This difference involves a change attitude, prior to the Pars prima and perhaps posterior to the De potentia, raising the question of the initial Thomist view.

In earlier works, then, the theory of causation seems to have been worked out on the analogy of the familiar distinction between the [being towards] and the [being in] of the relation. In action one has to distinguish between a formal content described as [from an agent] or [as proceeding from an agent to another], and on the other hand, a reality, substantial or accidental, termed the [principle of action] or the [cause of action] or even loosely actio. This terminology is to be found no less in the commentary on the Sentences than in the De potentia, but at least in the latter work it also is quite clear that the formal content is no more than a notional entity. In the two passages quoted below, the reader will be able to verify the following six propositions: (A) change from rest to activity is change in an improper and metaphorical sense; (B) the reverse change from activity to rest takes place without any real change in the agent; (C) when the agent is acting there is no composition of agent and action; (D) what remains unchanged is the [principle] or [cause of action]; (E) what comes and goes without changing the agent is the formal content, [from an agent]; (F) the analysis holds even in the case of a created agent such as fire.

And so a relation is something inhering (in a subject), though that does not result from the mere fact that it is a relation; as action, too, from the fact that it is action, is considered as from an agent, but as an accident it is considered as in the acting subject. And therefore, there is nothing to prevent an accident of this kind (B) from ceasing to be without (involving) a change of that (subject) in which it is, because its being is not realized insofar as it is in that subject, but insofar as it passes on to another; with the removal of that (passing on), the being of this accident is removed (E) in what regards the act but remains (D) in what regards the cause; as is the case also when, with the removal of the material (to be heated), the heating (F) is removed, though the cause of heating remains (De potentia, q. 7, a. 9, ad 7m)

But that which is attributed to something as proceeding from it to something else does not enter into composition with it, as (C) neither does action (enter into composition) with the agent… without any change in that which is related to another, a relation can cease to be through the change alone of the other; as also is clear about action (B), that there is no movement as regards action except metaphorically and improperly; as we say that (A) one passing from leisure to act is changed; which would not be the case if relation or action signified something remaining in the subject (Ibid. a. 8 c.)

If our interpretation of these passages is correct, then at least in the De potentia St Thomas had arrived at a theory of action that was in essential agreement with Aristotle’s. Evidently the two terminologies differ completely: on the Aristotelian view action is a relation of dependence in the effect; on the Thomist view action is a formal content attributed to the cause as causing. But these differences only serve to emphasize the fundamental identity of the two positions: both philosophers keenly realized that causation must not be thought to involve any real change in the cause as cause; Aristotle, because he conceived action as a motion, placed it in the effect; St Thomas, who conceived it simply as a formal content, was able to place it in the cause; but though they proceed by different routes, both arrive at the same goal, namely, that the objective difference between [to be able to act] and [to actually act] is attained without any change emerging in the cause as such.

This real agreement in terminological difference solves the problem of St Thomas’s thought on causation. John of St Thomas listed the passages in which action is placed, now in the agent and now in the recipient; from this he drew the conclusion that action, according to St Thomas, was inchoatively in the agent and perfectively in the recipient. But in point of fact St Thomas simply had two ways of saying that action involved no new entity in the agent; and so far was he from differing really from Aristotle that he seems to have been quite unaware of even his terminological departure from the Aristotelian position. This latter fact not only solves Cajetan’s perplexity over the apparent divergence between the commentary on the Physics and regular Thomist usage but also provides the most conclusive evidence against such as position of Billuart’s that a real distinction in the agent between [power to act] and [the act itself] is one of the pillars of Thomist thought.