Plato: Republic

(from Republic page 13)

Socrates: "You would argue that that the good are our friends and the bad our enemies?"
Polemarchus: "Yes:
S: "And instead of saying simply as we did at first, that it is just to do good to our friends and harm to our enemies, we should further say: It is just to do good to our friends when they are good, and harm to our enemies when they are enemies?"
P: "Yes, that appears to me to be the truth."
S: "But ought the just to injure anyone at all?"
P: "Undoubtedly he ought to injure those who are both wicked and his enemies.
S: "When horse are injured, are they improved or deteriorated?
P: "the latter."
S: "Deteriorated, that is to say, in the good qualities of horses, and not dogs?"
P: "Yes, of horses."
S: "And dogs are deteriorated in the good qualities of dogs, and not of horses?"
P: "Of course."
S: "And will not men who are injured be deteriorated in that which is the proper virtue of man?"
P: "Certainly"
S: "And that human virtue is justice?
P: "To be sure."
S. "Then men who are injured are of necessity made unjust?"
P: "That is the result."
S: "But can the musician by his art make men unmusical?"
P: "Certainly not."
S: "Or the horseman by his art make them bad horsemen?"
P: " Impossible."
S: " And can the just by justice make men unjust, or speaking generally, can the good by virtue make them bad?"
P: "Assurdly not."
S: "Any more than heat can produce cold?"
P: "It cannot."
S: "Or drought moisture?"
P: "Clearly not"
S: "Nor can the good harm anyone?"
P: "Impossible."
S: "And the just is good?"
P: "Certainly."
S: "Then to injure a friend or anyone else is not the act of a just man, but of the opposite, who is the unjust?"
P: " I think that what you say is quite true, Socrates."
S: "Then, is a man says that justice consists in the repayment of debts, and the good is the debt which a just man owes to his friends, and evil the debt which he owes to his enemies- to say this is not wise; for it is not true, if, as has been clearly shown, the injuring of another can be in no case just."