Nicene and Post-Nicene Fathers, Vol. IV:
Writings in Connection with the Manichæan Controversy.: Chapter 27

Chapter 27.—The Beauty of the Straight Line Might Be Taken from the Region of Darkness Without Taking Anything from Its Substance.  So Evil Neither Takes from Nor Adds to the Substance of the Soul.  The Straightness of Its Side Would Be So Far a Good Bestowed on the Region of Darkness by God the Creator.

29.  What is to be done with unhappy minds, perverse in error, and held fast by custom?  These men do not know what they say when they say those things; for they do not consider.  Listen to me; no one forces you, no one quarrels with you, no one taunts you with past errors, unless some one who has not experienced the divine mercy in deliverance from error:  all we desire is that the errors should some time or other be abandoned.  Think a little without animosity or bitterness.  We are all human beings:  let us hate, not one another, but errors and lies.  Think a little, I pray you.  God of mercy, help them to think, and kindle in the minds of inquirers the true light.  If anything is plain, is not this, that right is better than wrong?  Give me, then, a calm and quiet answer to this, whether making crooked the right line of the region of darkness which joins on to the right line of the region of light, would not detract from its beauty.  If you will not be dogged, you must confess that not only is beauty taken from it by its being made crooked, but also the beauty which it might have had from connection with the right line of the region of light.  Is it the case, then, that in this loss of beauty, in which right is made crooked, and harmony becomes discord, and agreement disagreement, there is any loss of substance?  Learn, then, from this that substance is not evil; but as in the body, by change of form for the worse, beauty is lost, or rather lessened, and what was called fair before is said to be ugly, and what was pleasing becomes displeasing, so in the mind the seemliness of a right will, which makes a just and pious life, is injured when the will changes for the worse; and by this sin the mind becomes miserable, instead of enjoying as before the happiness which comes from the ornament of a right will, without any gain or loss of substance.

30.  Consider, again, that though we admit that the border of the region of darkness was evil for other reasons, such as that it was dim and dark, or any other reason, still it was not evil in being straight.  So, if I admit that there was some evil in its color, you must admit that there was some good in its straightness.  Whatever the amount of this good, it is not allowable to attribute it to any other than God the Maker, from whom we must believe that all good in whatsoever nature comes, if we are to escape deadly error.  It is absurd, then, to say that this region is perfect evil, when in its straightness of border is found the good of not a little beauty of a material kind; and also to make this region to be altogether estranged, from the almighty and good God, when this good which we find in it can be attributed to no other but the author of all good things.  But this border, too, we are told, was evil.  Well, suppose it evil:  it would surely have been worse had it been crooked instead of straight.  And how can that be the perfection of evil than which something worse than itself can be thought of?  And to be worse implies that there is some good, the want of which makes the thing worse.  Here the want of straightness would make the line worse.  Therefore its straightness is something good.  And you will never answer the question whence this goodness comes, without reference to Him from whom we must acknowledge that all good things come, whether small or great.  But now we shall pass on from considering this border to something else.