Nicene and Post-Nicene Fathers, Vol. II:
City of God: Chapter 7

Chapter 7.—That We Ought Not to Expect to Find Any Efficient Cause of the Evil Will.

Let no one, therefore, look for an efficient cause of the evil will; for it is not efficient, but deficient, as the will itself is not an effecting of something, but a defect.  For defection from that which supremely is, to that which has less of being,—this is to begin to have an evil will.  Now, to seek to discover the causes of these defections,—causes, as I have said, not efficient, but deficient,—is as if some one sought to see darkness, or hear silence.  Yet both of these are known by us, and the former by means only of the eye, the latter only by the ear; but not by their positive actuality, 531 but by their want of it.  Let no one, then seek to know from me what I know that I do not know; unless he perhaps wishes to learn to be ignorant of that of which all we know is, that it cannot be known.  For those things which are known not by their actuality, but by their want of it, are known, if our expression may be allowed and understood, by not knowing them, that by knowing them they may be not known.  For when the eyesight surveys objects that strike the sense, it nowhere sees darkness but where it begins not to see.  And so no other sense but the ear can perceive silence, and yet it is only perceived by not hearing.  Thus, too, our mind perceives intelligible forms by understanding them; but when they are deficient, it knows them by not knowing them; for “who can understand defects?” 532

Footnotes

Specie.

Ps. 19.12.