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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 necessarily 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