And I kept seeking for an answer to the question, Whence is evil? And I sought it in an evil way, and I did not see the evil in my very search. I marshaled before the sight of my spirit all creation: all that we see of earth and sea and air and stars and trees and animals; and all that we do not see, the firmament of the sky above and all the angels and all spiritual things, for my imagination arranged these also, as if they were bodies, in this place or that. And I pictured to myself thy creation as one vast mass, composed of various kinds of bodies--some of which were actually bodies, some of those which I imagined spirits were like. I pictured this mass as vast--of course not in its full dimensions, for these I could not know--but as large as I could possibly think, still only finite on every side. But thou, O Lord, I imagined as environing the mass on every side and penetrating it, still infinite in every direction--as if there were a sea everywhere, and everywhere through measureless space nothing but an infinite sea; and it contained within itself some sort of sponge, huge but still finite, so that the sponge would in all its parts be filled from the immeasurable sea.180
Thus I conceived thy creation itself to be finite, and filled by thee, the infinite. And I said, “Behold God, and behold what God hath created!” God is good, yea, most mightily and incomparably better than all his works. But yet he who is good has created them good; behold how he encircles and fills them. Where, then, is evil, and whence does it come and how has it crept in? What is its root and what its seed? Has it no being at all? Why, then, do we fear and shun what has no being? Or if we fear it needlessly, then surely that fear is evil by which the heart is unnecessarily stabbed and tortured--and indeed a greater evil since we have nothing real to fear, and yet do fear. Therefore, either that is evil which we fear, or the act of fearing is in itself evil. But, then, whence does it come, since God who is good has made all these things good? Indeed, he is the greatest and chiefest Good, and hath created these lesser goods; but both Creator and created are all good. Whence, then, is evil? Or, again, was there some evil matter out of which he made and formed and ordered it, but left something in his creation that he did not convert into good? But why should this be? Was he powerless to change the whole lump so that no evil would remain in it, if he is the Omnipotent? Finally, why would he make anything at all out of such stuff? Why did he not, rather, annihilate it by his same almighty power? Could evil exist contrary to his will? And if it were from eternity, why did he permit it to be nonexistent for unmeasured intervals of time in the past, and why, then, was he pleased to make something out of it after so long a time? Or, if he wished now all of a sudden to create something, would not an almighty being have chosen to annihilate this evil matter and live by himself--the perfect, true, sovereign, and infinite Good? Or, if it were not good that he who was good should not also be the framer and creator of what was good, then why was that evil matter not removed and brought to nothing, so that he might form good matter, out of which he might then create all things? For he would not be omnipotent if he were not able to create something good without being assisted by that matter which had not been created by himself.
Such perplexities I revolved in my wretched breast, overwhelmed with gnawing cares lest I die before I discovered the truth. And still the faith of thy Christ, our Lord and Saviour, as it was taught me by the Catholic Church, stuck fast in my heart. As yet it was unformed on many points and diverged from the rule of right doctrine, but my mind did not utterly lose it, and every day drank in more and more of it.
http://www.ccel.org/ccel/augustine/confessions.x.html