"Then what does the skill we can call justice supply and to whom?"
"If we are to be consistent, Socrates, it must be the skill that enables us to help one's friends and injure one's enemies."
"So Simonides says that justice is to benefit one's friends and harm one's enemies?"
"I think so."
"Who then is best able to benefit his friends and harm his enemies in matters of health?"
"A doctor."
"And in the risks of a sea voyage?"
"A navigator."
"And what about a just man? In what activity or occupation will he best be able to help his friends and harm his enemies?"
"In war: he will fight against his enemies and for his friends.."
"Good. Yet people who are healthy have no use for a physician, have they, Polemarchus?"
"True."
"Nor those that stay on land of a navigator?"
"No."
"Do you then maintain that those who are not at war have no use for a just man?"
(From Plato's "Republic" 1.1.2d)