St. Augustine

On happiness...

That all men which to be happy is a certitude for anyone who can think. But, so long as human intelligence remains incapable of deciding which men are happy and how they become so, endless controversies arise in which philosophers waste their time and toil.

On the origin of sin...

If this evil will existed in no nature, then it did not exist at all. If it existed in some nature, then it vitiated, corrupted, injured that nature and, therefore, deprived it of some good. An evil will could not exist in an evil nature but only in a good one, mutable enough to suffer harm from this deprivation. For, if no harm were done, then there was no deprivation and, consequently, no right to call the will evil. But, if harm was done, it was done by destroying or diminishing what was good. Thus, an evil will could not have existed from all eternity in a nature in which a previously existing good had to be eliminated before the evil will could harm the nature. But, if it did no exist from all eternity, who, then, caused this evil will?
The only remaining suggesting is that the cause of the evil will was something which had no will. My next question is whether this "something" was superior, inferior or equal to the will. If superior, then it was better. So, then, how can it have the same: for, as long as two wills are equally good, one cannot produce an evil will in the other. The supposition remains, then, that it was an inferior thing without a will which produced the evil will of the angelic nature which first sinned.
But that thing itself, whatever it was, even though it was low to the lowest point of earthliness, was, without doubt good since it was a nature and a being having its own character and species in its own genus and order. How, I ask, can good be the cause of evil? For, when the will, abandoning what is above it, turns itself to something lower, it becomes evil because the very turning itself and not the thing to which it turns is evil. Therefore, an inferior being does not make the will evil but the will itself, because it is a created will, wickedly and inordinately seeks the inferior being...Take a person who says that the one who consents to the temptation and enticement made his own will evil although previously he had been entirely good. Recall the facts. The one (the evil man) consents, while the other (the good man) does not, to a sinful desire concerning a beautiful person; the beauty was seen by both equally, and before the temptation both men were absolutely aling in mind and body. Now, the person who talks of a man making his own will evil must ask why the man made his will evil, whether because he is a nature or because he is nature made out of nothing? He will learn that the evil arise not from the fact that the man is a nature, but from the fact that the nature was made out of nothing.

-City of God, St. Augustine

One of my favorite books