In this book we have a continuation of the discussion concerning friendship which occupied the greater portion of Book VIII. Friendship was, in Aristotle's opinion, one of the most important achievements in the lives of good people. Its benefits were not confined to the individuals between whom the friendships were formed but they extended to the whole of society. It was therefore important for the student of ethics to understand the true nature of friendship and to see how it is related to the many activities that are carried on by the members of any given society. This was undoubtedly the reason he found it necessary to devote so much time and space to a consideration of the many questions that arise in connection with it.
Because friendship at its best is a kind of spontaneous activity in which one's motives with reference to the welfare of the other person is a predominant factor it is impossible to prescribe a definite set of rules which friends must follow under any and all circumstances. A true friend will understand the particular situation in which he needs to act and he will do that which he believes to be appropriate for promoting the best interests of the one he wishes to befriend. But important as the motive is in matters pertaining to friendship there are certain guide lines which ought to be observed and although these are necessarily of a general nature they will help one to determine the appropriate thing to do in the particular instances which arise. It was for the purpose of setting forth these guide lines that the instruction recorded in this book was given. For example, there are certain considerations which one should have in mind for determining the extent to which friends are obligated toward one another. The obligation will of course vary according to the nature of that which one does for the sake of his friend. The effect which the matter of giving produces both upon the giver and the recipient must also be taken into account for there is a strong tendency for one to overestimate the value of his own good deeds while they do not appear in the same light to the person for whom they were done. Again, there are situations in which the obligations which one recognizes appear to be in conflict with one another, and it is necessary to establish the order of preference which should be followed. Further considerations need to be given for determining the qualities which make for lasting friendships as well as the factors which tend to destroy it. On all of these points Aristotle indicates in a general way the principles which should be followed but it remains for the individual to determine for himself the precise method in which they are applied in particular instances.
One of the most important problems discussed in this book is the extent to which one should pursue his own individual interests and when, if ever, he should sacrifice his own interests in order to promote the welfare of other persons. This has always been a controversial issue throughout the history of ethics. The view that man is essentially a selfish creature and whatever he does is an expression of this motive was a fairly common one among the ancient Greeks. Many of the characters in Plato's dialogues represent this position. On the other hand Socrates and several of his followers taught that man lives at his best only when he subordinates his own private interests to the welfare of the society in which he lives. However, the question still remains as to whether one works for the interests of others merely as a means for promoting his own welfare or whether he does it purely for the sake of the other person regardless of any benefits which may be derived from it for himself. It is a difficult question and one that can be resolved only through a clarification of the terms that are used. If selfishness is to be condemned one must know precisely what it means to be selfish and if altruism is to be approved one must be able to distinguish between actions which are selfish and the ones which are altruistic. Whether the same action can be both selfish and altruistic at the same time must also be considered. In other words, is it possible to harmonize self-love and love of others?
Aristotle's treatment of this topic consists in his attempt to combine in a harmonious manner the truth that is included in each of the apparently opposing views. He recognized that there is a sense in which it is true that each person not only does pursue his own interest but he ought to do so. At the same time it is also true that one ought to pursue the best interests of others even though this may require that he sacrifice his own interests in order to reach this objective. The solution to this apparent paradox is found by making a distinction between two kinds of self interest. There is the kind of self-love which excludes the welfare of others and there is the kind the is inclusive of it. The former is the type of selfishness which should be condemned and the latter one should be approved. In fact it is the latter type which coincides with what is usually meant by altruism. When one identifies his own interests with the welfare of others he is realizing a larger and more inclusive self and it is this type of selfhood which constitutes his real self or what is commonly known as one's ideal self.