Part 1425

ts 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 (Q[45], A[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 corporea