The friendship of siblings and kinsmen is like that of comrades. Friendship between parents and children involves much more pleasure and usefulness than other friendships because of their life in common. Friendship between a husband and wife exists by nature for men and women tend to form couples by nature for the sake of reproduction and for supplying each other's needs. Children tend to keep a marriage together, because they are a good common to both spouses. Quarrels occur most of all in friendships based on usefulness because each is only using the other for his own benefit, but in friendships based on virtue quarrels are rare because the friends are eager to treat each other well.
Friendships based on usefulness can be ethical or legal.
Legal friendships are formed based on specified forms of exchange, while ethical ones do not have specified terms by reciprocity in giving is expected. A person should not receive a service in this kind of friendship unless he is able to repay it. In friendships based on virtue quarrels over who receives more do not arise because intention is the measure. The claim of each person is correct, and it is possible to fulfill both claims because the superior should receive more honor and the needy should receive more material gain. Thus in associations of unequals the party who is benefitted should repay the superior party with honor, because it is impossible to give an equal amount in return.