St. Thomas imputes real causality to creatures, but he places it within a very strict limit: the creature can be directly responsible for the becoming of this accident or this substance, but God alone is responsible for the existence of the same particular.
Every effect depends on its cause, so far as it is itscause. But we must observe that an agent may be the cause of the “becoming” of its effect, but not directly of its “being.” This may be seen both in artificial and innatural beings: for the builder causes the house in its “becoming,” but he is not the direct cause of its “being.” For it is clear that the “being” of the house is a result of its form, which consists in the putting together and arrangement of the materials, and results from the natural qualities of certain things. Thus a cook dresses the food by applying the natural activity of fire; thus a builder constructs a house, by making use of cement, stones, and wood which are able to be put together in a certain order and to preserve it. Therefore the “being” of a house depends on the nature of these materials, just as its “becoming” depends on the action of the builder. The same principle applies to natural things. For if an agent is not the cause of a form as such, neither will it be directly the cause of “being” which results from that form; but it will be the cause of the effect, in its “becoming” only.
St. Thomas then shows why even natural generation is directly responsible for becoming and not existence.
Now it is clear that of two things in the same species one cannot directly cause the other’s form as such, since it would then be the cause of its own form, which isessentially the same as the form of the other; but it can be the cause of this formfor as much as it is in matter–in other words, it may be the cause that “this matter” receives “this form.” And this is to be the cause of “becoming,” as when man begets man, and fire causes fire. Thus whenever a natural effect is such that it has an aptitude to receive from its active cause an impression specifically the same as in that active cause, then the “becoming” of the effect, but not its “being,” depends on the agent.
Applied to the spiritual life, this teaching speaks to our awareness that while we are certainly respnsible for doing things, that anything should come of our actions requires divine assistance:
The man’s mind thinks out his way, but God directs his steps (Proverbs, 16:9)
Man shoes the horse, but to God is the victory
I planted, Apollos watered, but God gave the growth (1 Cor, 3:6).
One of the most beautiful and exact descriptions of this is in The Imitation, Book III chap. 2
You speak, O Lord God, Who inspired and enlightened all the prophets; for You alone, without them, can instruct me perfectly, whereas they, without You, can do nothing. They, indeed, utter fine words, but they cannot impart the spirit. They do indeed speak beautifully, but if You remain silent they cannot inflame the heart. They deliver the message; You lay bare the sense. They place before us mysteries, but You unlock their meaning. They proclaim commandments; You help us to keep them. They point out the way; You give strength for the journey. They work only outwardly; You instruct and enlighten our hearts. They water on the outside; You give the increase.
They cry out words; You give understanding to the hearer.
Let not Moses speak to me, therefore, but You, the Lord my God, everlasting truth, speak lest I die and prove barren if I am merely given outward advice and am not inflamed within