Augustine of Hippo, On Man's Perfection in Righteousness Chapter 2: The First Breviate of Cœlestius

I. “First of all,” says he, “he must be asked who denies man’s ability to live without sin, what every sort of sin is,—is it such as can be avoided? or is it unavoidable? If it is unavoidable, then it is not sin; if it can be avoided, then a man can live without the sin which can be avoided. No reason or justice permits us to designate as sin what cannot in any way be avoided.” Our answer to this is, that sin can be avoided, if our corrupted nature be healed by God’s grace, through our Lord Jesus Christ. For, in so far as it is not sound, in so far does it either through blindness fail to see, or through weakness fail to accomplish, that which it ought to do; “for the flesh lusteth against the spirit, and the spirit against the flesh,” so that a man does not do the things which he would.