Aristotle, Eudemian Ethics

Friendship—its nature and qualities, what constitutes a friend, and whether the term friendship has one or several meanings, [20] and if several, how many, and also what is our duty towards a friend and what are the just claims of friendship—is a matter that calls for investigation no less than any of the things that are fine and desirable in men's characters. For to promote friendship is thought to be the special task of political science; and people say that it is on this account that goodness is a valuable thing, for persons wrongfully treated by one another cannot be each other's friends. Furthermore we all say that justice and injustice are chiefly displayed towards friends; it is thought that a good man is a friendly man, and that friendship is a state of the moral character; and if one wishes to make men not act unjustly, it is enough to make them friends, for true friends do not wrong one another. But neither will men act unjustly if they are just; therefore justice and friendship are either the same or nearly the same thing.

In addition to this, we consider a friend to be one of the greatest goods, and friendlessness and solitude a very terrible thing, because the whole of life and voluntary association is with friends;