The Republic by Plato Part 1 Page 16

for he thinks that a friend ought always to do good to a friend and never evil.

You mean that the return of a deposit of gold which is to the injury of the receiver, if the two parties are friends, is not the repayment of a debt, — that is what you would imagine him to say?

Yes.

And are enemies also to receive what we owe to them?

To be sure, he said, they are to receive what we owe them, and an enemy, as I take it, owes to an enemy that which is due or proper to him — that is to say, evil.

Simonides, then, after the manner of poets, would seem to have spoken darkly of the nature of justice; for he really meant to say that justice is the giving to each man what is proper to him, and this he termed a debt.