If, in fact, the brightness of the visible sun is not affected by the gloom and darkness of the grave, if it is not defiled by filth from the sewers, is there any wonder that the most high and infinite Spirit should touch ever so lightly with his splendor the dark and squalid hearts of certain men, only to remain clean and unsullied in his own purity? Anyone who ordains, therefore, and is guilty of any crime--whether he is proud, or lustful, whether he is a murderer, or even a simonist--he is, indeed, tainted and undoubtedly steeped in deadly leprosy, but the gift of God that is passed on through him is defiled by no one's corruption, nor infected by anyone's disease. That which flows through the minister is pure, and passes to a fertile soil, clean and limpid. Holy Church, to be sure, is a garden of delights, a spiritual paradise, watered by a river of heaven's choicest gifts. Let us assume, therefore, that wicked priests are like channels made of stone; in channels of stone water makes nothing grow until after flowing through them it pours out on fertile, cultivated fields. Even though the passing years should successively produce many unworthy priests, so that both they that ordain and they that are ordained are found equally corrupt, this living fountain is, nevertheless, is not restrained from flowing through the glade of the Church to the end of time, and from this fountain not only the ranks of the priesthood but all who are reborn in Christ raise to their lips the cup of salvation. Through priests, to be sure, baptism and holy oil come to us, as well as every dispensing of the sacraments of the Church.
[St. Peter Damian, Letter 40, Peter Damian: Letters 31-60, Blum, tr., Catholic University of America Press (Washington, DC: 1990) pp. 139-140. This letter is more commonly known as the Liber gratissimus, a title given it by St. Peter himself, and it is the most important Latin discussion of ordination prior to the thirteenth century.]
The image of stone channels is drawn from St. Augustine, although St. Peter is deliberately generalizing the point, since Augustine in context is only considering the integrity of baptism. Whereas Augustine was addressing people who were insisting on rebaptism of those who had had Christian baptism, Peter is addressing people who were insisting on reordination of those who had become ordained through simony; earlier in the work he argues that baptism and orders are closely connected in how they distribute grace. From Augustine's Tractates on the Gospel of John, Tractate 5.15:
[H]e who is a proud minister is reckoned with the devil; but the gift of Christ is not contaminated, which flows through him pure, which passes through him liquid, and comes to the fertile earth. Suppose that he is stony, that he cannot from water rear fruit; even through the stony channel the water passes, the water passes to the garden beds; in the stony channel it causes nothing to grow, but nevertheless it brings much fruit to the gardens. For the spiritual virtue of the sacrament is like the light: both by those who are to be enlightened is it received pure, and if it passes through the impure it is not stained. Let the ministers be by all means righteous, and seek not their own glory, but His glory whose ministers they are; let them not say, The baptism is mine; for it is not theirs.