Aristotle distinguishes three forms of friendship according to what motivates them. Friendships of utility are held together by mutual benefit — when the benefit ceases, the friendship dissolves. Friendships of pleasure are held together by enjoyment of each other's company — common among the young, and equally temporary. Both are genuine friendships, but incomplete ones.
The highest form is friendship between people who are good and similar in virtue. These friends value each other not for what they receive but for what the other is. Because character is stable, such friendships endure. Because each genuinely wishes the other well, they call forth the best in each other — spending time together, sharing their lives, taking pleasure in each other's activities. This is friendship in the fullest sense.
Aristotle's deepest claim about friendship is that a genuine friend is a second self. To love a friend in the proper sense is to extend to them the same concern for flourishing that the good person properly has for themselves. This connection between self-love and friendship is characteristic of Aristotle's approach: the person who does not properly value their own rational nature cannot genuinely value it in another.
Friendship is not merely a private matter. Aristotle explicitly connects it to justice and to the bonds that hold political communities together. The goodwill, mutual recognition, and concern for the other's well-being that characterise friendship are also the dispositions that make civic life possible. A city of strangers who merely tolerate each other is a diminished community compared to one whose citizens share the kind of concord that Aristotle identifies with civic friendship.
Friendship is treated across Books VIII and IX of the Nicomachean Ethics, in what remains the most comprehensive philosophical treatment of philia in the ancient world.