the cause of
evil.

Obj. 3: Further, a deficient effect can proceed only from a deficient
cause. But evil is a deficient effect. Therefore its cause, if it has
one, is deficient. But everything deficient is an evil. Therefore the
cause of evil can only be evil.

Obj. 4: Further, Dionysius says (Div. Nom. iv) that evil has no
cause. Therefore good is not the cause of evil.

_On the contrary,_ Augustine says (Contra Julian. i, 9): "There is no
possible source of evil except good."

_I answer that,_ It must be said that every evil in some way has a
cause. For evil is the absence of the good, which is natural and due
to a thing. But that anything fail from its natural and due
disposition can come only from some cause drawing it out of its proper
disposition. For a heavy thing is not moved upwards except by some
impelling force; nor does an agent fail in its action except from some
impediment. But only good can be a cause; because nothing can be a
cause except inasmuch as it is a being, and every being, as such, is
good.

And if we consider the special kinds of causes, we see that the
agent, the form, and the end, import some kind of perfection which
belongs to the notion of good. Even matter, as a potentiality to
good, has the nature of good. Now that good is the cause of evil by
way of the material cause was shown above (Q. 48, A. 3). For it was
shown that good is the subject of evil. But evil has no formal cause,
rather is it a privation of form; likewise, neither has it a final
cause, but rather is it a privation of order to the proper end; since
not only the end has the nature of good, but also the useful, which
is ordered to the end. Evil, however, has a cause by way of an agent,
not directly, but accidentally.

In proof of this, we must know that evil is caused in the action
otherwise than in the effect. In the action evil is caused by reason
of the defect of some principle of action, either of the principal or
the instrumental agent; thus the defect in the movement of an animal
may happen by reason of the weakness of the motive power, as in the
case of children, or by reason only of the ineptitude of the
instrument, as in the lame. On the other hand, evil is caused in a
thing, but not in the proper effect of the agent, sometimes by the
power of the agent, sometimes by reason of a defect, either of the
agent or of the matter. It is caused by reason of the power or
perfection of the agent when there necessaril