Aristotle proposes a definition of friendship in Book VIII of Nicomachean Ethics. He says that friendship both is a virtue and involves the exercise of virtue, and he states that it is a necessity in life. In friendship, people are able to exercise virtue together in a community. Friendship is also a kind of excellence, or virtue, which one can practice. No one, however, is able to live a fully human life without friends, as man is by nature a social animal.
First, Aristotle argues that friendship requires mutual affection. You may have affection for a painting or a memory, but you are not friends with these things because they do not return the affection. If there is no mutual affection between two parties, then there is no friendship either. Aristotle assumes that friendship involves affection, but he argues that affection must be reciprocated in order for there to be friendship.
Second, Aristotle proposes that friendship depends upon reciprocated goodwill. You may wish the best for someone, but if they hope to harm you, then you are not friends. Again, Aristotle assumes that friendship involves goodwill, and then argues that the goodwill must be returned in order for a relationship to be friendship. Thus reciprocated goodwill is a necessary condition for friendship.
Third, Aristotle shows that awareness of mutual feeling is necessary for friendship. Although you may have strong feelings for the frontman of your favorite band, no one would call you friends if he does not know who you are. Hence there must be mutual knowledge of feelings for a relationship to be a friendship.
Friendship, for Aristotle, is both a virtue and involves the exercise of virtue. It is a necessary part of the good life because no one can fully actualize their human nature without it. The necessary conditions for a relationship to be a friendship are mutual affection, reciprocated goodwill, and awareness of mutual feeling. In this way, Aristotle defines friendship.