St. Augustine on Waterboarding

"Even when a city is enjoying the profoundest peace, some men must be sitting in judgment on their fellow men. Even at their best, what misery and grief they cause! No human judge can read the conscience of the man before him. That is why so many innocent witnesses are tortured to find what truth there is in the alleged guilt of other men. It is even worse when the accused man himself is tortured to find out if he be guilty. Here a man still unconvicted must undergo certain suffering for an uncertain crime-- not because his guilt is known, but because his innocence is unproved. Thus it often happens that the ignorance of the judge turns to tragedy for the innocent party. There is something still more insufferable-- deplorable beyond all cleansing with our tears. Often enough, when a judge tries to avoid putting a man to death whose innocence is not manifest, he has him put to torture, and so it happens, because of woeful lack of evidence, that he both tortures and kills the blameless man whom he tortured lest he kill him without cause. And if, on Stoic principles, the innocent man chooses to escape from life rather than endure such torture any longer, he will confess to a crime he never committed. And when it is over, the judge will still be in the dark whether the man he put to death was guilty or not guilty, even though he tortured him to save his innocent life, and then condemned him to death. Thus, to gather evidence, he tortures an innocent man, and lacking evidence, kills him."

St. Augustine, The City of God, Book XIX, Chapter 6, pp. 444-445.