y follows on the form
intended by the agent the privation of another form; as, for instance,
when on the form of fire there follows the privation of the form of
air or of water. Therefore, as the more perfect the fire is in
strength, so much the more perfectly does it impress its own form, so
also the more perfectly does it corrupt the contrary. Hence that evil
and corruption befall air and water comes from the perfection of the
fire: but this is accidental; because fire does not aim at the
privation of the form of water, but at the bringing in of its own
form, though by doing this it also accidentally causes the other.
But if there is a defect in the proper effect of the fire--as, for
instance, that it fails to heat--this comes either by defect of the
action, which implies the defect of some principle, as was said above,
or by the indisposition of the matter, which does not receive the
action of the fire, the agent. But this very fact that it is a
deficient being is accidental to good to which of itself it belongs to
act. Hence it is true that evil in no way has any but an accidental
cause; and thus is good the cause of evil.

Reply Obj. 1: As Augustine says (Contra Julian. i): "The Lord calls
an evil will the evil tree, and a good will a good tree." Now, a good
will does not produce a morally bad act, since it is from the good
will itself that a moral act is judged to be good. Nevertheless the
movement itself of an evil will is caused by the rational creature,
which is good; and thus good is the cause of evil.

Reply Obj. 2: Good does not cause that evil which is contrary to
itself, but some other evil: thus the goodness of the fire causes
evil to the water, and man, good as to his nature, causes an act
morally evil. And, as explained above (Q. 19, A. 9), this is by
accident. Moreover, it does happen sometimes that one contrary causes
another by accident: for instance, the exterior surrounding cold
heats (the body) through the concentration of the inward heat.

Reply Obj. 3: Evil has a deficient cause in voluntary things
otherwise than in natural things. For the natural agent produces the
same kind of effect as it is itself, unless it is impeded by some
exterior thing; and this amounts to some defect belonging to it.
Hence evil never follows in the effect, unless some other evil
pre-exists in the agent or in the matter, as was said above. But in
voluntary things the defect of the action comes from the will
actuall