The friendships that give rise to complaints are confined to the Useful.

Such friendships involve a legal element of strict and measured reciprocity [mere trade], and a moral or unwritten understanding, which is properly friendship.

Each party is apt to give less and expect more than he gets; and the rule must be for each to reciprocate liberally and fully, in such manner and kind as they are able (XIII.).

In unequal friendships, between a superior and inferior, the inferior has the greater share of material assistance, the superior should receive the greater honour (XIV.).
Book Ninth proceeds without any real break.

It may not be always easy to fix the return to be made for services received.