On Friendship
By Aristotle (384-322 B.C)
As the motives to Friendship differ in kind, so do the respective feelings and Friendships.
The species then of Friendship are three, in number equal to the objects of it, since in the line of each there may be “mutual affection mutually known.”
Now they who have Friendship for one another desire one another’s good according to the motive of their Friendship; accordingly they whose motive is utility have no Friendship for one another really, but only insofar as some good arises to them from one another.
And they whose motive is pleasure are in like case: I mean，they have Friendship for men of easy pleasantry, not because they are of a given character but because they are pleasant to themselves…
Such Friendships are of course very liable to dissolution if the parties do not continue alike: I mean, that the others cease to have any Friendship for them when they are no longer pleasurable or useful…
That then is perfect Friendship which subsists between those who are good and whose similarity consists in their goodness: for these men wish one another’s good in similar ways; insofar as they are good (and good they are in themselves); and those are specially friends who wish good to their friends for their sakes, because they feel thus toward them on their own account and not as a mere matter of result; so the Friendship between these men continues to subsist so long as they are good; and goodness, we know, has in it a principle of permanence…
Rare it is probable Friendships of this kind will be, because men of this kind are rare.
Besides, all requisite qualifications being presupposed, there is further required time and intimacy: for, as the proverb says, men cannot know one another “till they have eaten the requisite quantity of salt together”; nor can they in fact admit one another to intimacy, much less be friends, till each has appeared to the other and been proved to be a fit object of Friendship.
They who speedily commence an interchange of friendly actions may be said to wish to be friends, but they are not so unless they are also proper objects of Friendship and mutually known to be such: that is to say, a desire for Friendship may arise quickly but not Friendship itself.
(From the Nicomachean Ethics)