But on the contingency of evil being infinitely intensified, and good being continually diminished by that intensification of evil, it appears that good may be diminished by evil even to infinity. And the good so diminished must be finite, for infinite good is not capable of evil. It seems then that in time good may be entirely taken away by evil.
This then is the reply. Evil, as we have seen, entirely takes away the good to which it is opposed, as blindness takes away sight: but there must remain that good which is the subject of evil, which subject, as such, bears a character of goodness, inasmuch as it is in potentiality to the actuality of good, whereof it is deprived by evil.* The less then it is in potentiality to that good, the less good it will be. But a subject comes to be less in potentiality, or openness to a form, not only by the subtraction of some part of its potentiality, which is tantamount to subtraction of part of the subject itself, but also by the said potentiality being impeded by some contrary act from issuing in the actuality of the form.* Good therefore is diminished by evil rather by the planting of evil, its contrary, than by the taking off of any portion of good. And this agrees with what has been already said about evil: for we said that evil happens beside the intention of the doer, who always intends some good, and upon the good intended there follows the exclusion of some other good opposite to that good. The greater then the multiplication of that good, upon which, contrary to the intention of the agent, evil follows, the greater the diminution of potentiality in respect of the opposite good; and so all the more may good be said to be diminished by evil. This diminution however of good by evil in the physical world cannot go on indefinitely: for all the physical forms and powers are limited, and come to some term beyond which they cannot go.* But in moral matters this diminution of good by evil may proceed to infinity: for the understanding and the will have no limits to their acts: thus he who wills to commit a theft may will it again and commit another, and so to infinity. The further then the will tends towards undue ends, the more difficult it becomes for it to return to its proper and due end, as may be seen in persons in whom the habit of vice has been induced by a custom of sinning. Thus then by moral evil the good of natural aptitude may be diminished without limit: yet it can never be totally taken away, but always waits on nature while that remains.
3.11 : That Evil is founded in some Good
3.14 : That Evil is an Accidental Cause