“But while in associations that involve exchange, what is just in this sense holds them together, it is something reciprocal by way of proportionality, and not by way of equality. For a city stays together by paying things back proportionately, since people seek either to pay back evil, and if they cannot, that seems to be slavery, or to pay back good, and if they cannot do that, exchange does not happen, and they stay together by means of exchange.” (Aristotle: Nicomachean Ethics, 88)
