No one punishes the evil-doer under the notion, or for the reason, that he has done wrong -- only the unreasonable fury of a beast acts in that way. But he who desires to inflict rational punishment does not retaliate for a past wrong, for that which is done cannot be undone, but he has regard to the future, and is desirous that the man who is punished, and he who sees him punished, may be deterred from doing wrong again.
Charmides. Lysis. Laches. Protagoras. Euthydemus. Cratylus. Phaedrus. Ion. Symposium (ed. 1892)