h 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 Obj. 1: This passage of Augustine is to be understood of the
whole corporeal nature considered as a whole, which 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 Obj. 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