Objection 1. It would seem that no bodies are active. For Augustine says (De Civ. Dei v, 9): "There are things that are acted upon, but do not act; such are bodies: there is one Who acts but is not acted upon; this is God: there are things that both act and are acted upon; these are the spiritual substances."
Objection 2. Further, every agent except the first agent requires in its work a subject susceptible of its action. But there is not substance below the corporeal substance which can be susceptible of the latter's action; since it belongs to the lowest degree of beings. Therefore corporeal substance is not active.
Objection 3. Further, every corporeal substance is limited by quantity. But quantity hinders substance from movement and action, because it surrounds it and penetrates it: just as a cloud hinders the air from receiving light. A proof of this is that the more a body increases in quantity, the heavier it is and the more difficult to move. Therefore no corporeal substance is active.
Objection 4. Further, the power of action in every agent is according to its propinquity to the first active cause. But bodies, being most composite, are most remote from the first active cause, which is most simple. Therefore no bodies are active.
Objection 5. Further, if a body is an agent, the term of its action is either a substantial, or an accidental form. But it is not a substantial form; for it is not possible to find in a body any principle of action, save an active quality, which is an accident; and an accident cannot be the cause of a substantial form, since the cause is always more excellent than the effect. Likewise, neither is it an accidental form, for "an accident does not extend beyond its subject," as Augustine says (De Trin. ix, 4). Therefore no bodies are active.
On the contrary, Dionysius says (Coel. Hier. xv) that among other qualities of corporeal fire, "it shows its greatness in its action and power on that of which it lays hold."
I answer that, It is apparent to the senses that some bodies are active. But concerning the action of bodies there have been three errors. For some denied all action to bodies. This is the opinion of Avicebron in his book on The Fount of Life, where, by the arguments mentioned above, he endeavors to prove that no bodies act, but that all the actions which seem to be the actions of bodies, are the actions of some spiritual power that penetrates all bodies: so that, according to him, it is not fire that heats, but a spiritual power which penetrates, by means of the fire. And this opinion seems to be derived from that of Plato. For Plato held that all forms existing in corporeal matter are participated thereby, and determined and limited thereto; and that separate forms are absolute and as it were universal; wherefore he said that these separate forms are the causes of forms that exist in matter. Therefore inasmuch as the form which is in corporeal matter is determined to this matter individualized by quantity, Avicebron held that the corporeal form is held back and imprisoned by quantity, as the principle of individuality, so as to be unable by action to extend to any other matter: and that the spiritual and immaterial form alone, which is not hedged in by quantity, can issue forth by acting on something else.
But this does not prove that the corporeal form is not an agent, but that it is not a universal agent. For in proportion as a thing is participated, so, of necessity, must that be participated which is proper thereto; thus in proportion to the participation of light is the participation of visibility. But to act, which is nothing else than to make something to be in act, is essentially proper to an act as such; wherefore every agent produces its like. So therefore to the fact of its being a form not determined by matter subject to quantity, a thing owes its being an agent indeterminate and universal: but to the fact that it is determined to this matter, it owes its being an agent limited and particular. Wherefore if the form of fire were separate, as the Platonists supposed, it would be, in a fashion, the cause of every ignition. But this form of fire which is in this corporeal matter, is the cause of this ignition which passes from this body to that. Hence such an action is effected by the contact of two bodies.
But this opinion of Avicebron goes further than that of Plato. For Plato held only substantial forms to be separate; while he referred accidents to the material principles which are "the great" and "the small," which he considered to be the first contraries, by others considered to the "the rare" and "the dense." Consequently both Plato and Avicenna, who follows him to a certain extent, held that corporeal agents act through their accidental forms, by disposing matter for the substantial form; but that the ultimate perfection attained by the introduction of the substantial form is due to an immaterial principle. And this is the second opinion concerning the action of bodies; of which we have spoken above when treating of the creation (45, 8).
The third opinion is that of Democritus, who held that action takes place through the issue of atoms from the corporeal agent, while passion consists in the reception of the atoms in the pores of the passive body. This opinion is disproved by Aristotle (De Gener. i, 8,9). For it would follow that a body would not be passive as a whole, and the quantity of the active body would be diminished through its action; which things are manifestly untrue.
We must therefore say that a body acts forasmuch as it is in act, on a body forasmuch as it is in potentiality.
Reply to Objection 1. This passage of Augustine is to be understood of the whole corporeal nature considered as a whole, while thus has no nature inferior to it, on which it can act; as the spiritual nature acts on the corporeal, and the uncreated nature on the created. Nevertheless one body is inferior to another, forasmuch as it is in potentiality to that which the other has in act.
From this follows the solution of the second objection. But it must be observed, when Avicebron argues thus, "There is a mover who is not moved, to wit, the first maker of all; therefore, on the other hand, there exists something moved which is purely passive," that this is to be conceded. But this latter is primary matter, which is a pure potentiality, just as God is pure act. Now a body is composed of potentiality and act; and therefore it is both active and passive.
Reply to Objection 3. Quantity does not entirely hinder the corporeal form from action, as stated above; but from being a universal agent, forasmuch as a form is individualized through being in matter subject to quantity. The proof taken from the weight of bodies is not to the purpose.
First, because addition of quantity does not cause weight; as is proved (De Coelo et Mundo iv, 2).
Secondly, it is false that weight retards movement; on the contrary, the heavier a thing, the greater its movement, if we consider the movement proper thereto.
Thirdly, because action is not effected by local movement, as Democritus held: but by something being reduced from potentiality to act.
Reply to Objection 4. A body is not that which is most distant from God; for it participates something of a likeness to the Divine Being, forasmuch as it has a form. That which is most distant from God is primary matter; which is in no way active, since it is a pure potentiality.
Reply to Objection 5. The term of a body's action is both an accidental form and a substantial form. For the active quality, such as heat, although itself an accident, acts nevertheless by virtue of the substantial form, as its instrument: wherefore its action can terminate in a substantial form; thus natural heat, as the instrument of the soul, has an action terminating in the generation of flesh. But by its own virtue it produces an accident. Nor is it against the nature of an accident to surpass its subject in acting, but it is to surpass it in being; unless indeed one were to imagine that an accident transfers its identical self from the agent to the patient; thus Democritus explained action by an issue of atoms.
Source: Summa Theologica (New Advent)