Prisons as deterrents

“In punishing wrongdoers, no one concentrates on the fact that a man has done wrong in the past, or punishes him on that account, unless taking blind vengeance like a beast.  No, punishment is not inflicted by a rational man for the sake of the crime that has been committed–after all one cannot undo what is past–but for the sake of the future, to prevent either the same man or, by the spectacle of his punishment, someone else, from doing wrong again.  But to hold such a view amounts to holding that virtue can be instilled by education; at all events the punishment is inflicted as a deterrent.  This then is the view held by all who inflict it whether privately or publicly.” — Plato, Protagoras (trans. Guthrie)