Nicomachean Ethics IX.4

On neighbors, and loving them -- at least in the friendly way (philia, in the Greek, not eros or one of the several other 'love' words as we discussed in VIII.3). This is usually thought to be one of the most important chapters of the work.

Friendly relations with one's neighbours, and the marks by which friendships are defined, seem to have proceeded from a man's relations to himself.

This is the second hint of the definition of 'a friend' as 'another self,' which will become central to the concept. The first was in VIII.12. 

For (1) we define a friend as one who wishes and does what is good, or seems so, for the sake of his friend, or (2) as one who wishes his friend to exist and live, for his [i.e. the friend's own] sake; which mothers do to their children, and friends do who have come into conflict. And (3) others define him as one who lives with and (4) has the same tastes as another, or (5) one who grieves and rejoices with his friend; and this too is found in mothers most of all. It is by some one of these characteristics that friendship too is defined.

Now each of these is true of the good man's relation to himself (and of all other men in so far as they think themselves good; virtue and the good man seem, as has been said, to be the measure of every class of things).

This was also established in the pivotal I.3. I usually cite that to remind us 'to seek precision in different disciplines at the appropriate levels, but also, "Now each man judges well the things he knows, and of these he is a good judge[.]" Thus, the man who knows virtue and proves it by being virtuous is a good judge of virtue, or at least the particular virtue that he has (e.g. courage).   

For [the good man's] opinions are harmonious, and he desires the same things with all his soul; and therefore he wishes for himself what is good and what seems so, and does it (for it is characteristic of the good man to work out the good), and does so for his own sake (for he does it for the sake of the intellectual element in him, which is thought to be the man himself); and he wishes himself to live and be preserved, and especially the element by virtue of which he thinks. For existence is good to the virtuous man, and each man wishes himself what is good, while no one chooses to possess the whole world if he has first to become some one else (for that matter, even now God possesses the good); he wishes for this only on condition of being whatever he is; and the element that thinks would seem to be the individual man, or to be so more than any other element in him. And such a man wishes to live with himself; for he does so with pleasure, since the memories of his past acts are delightful and his hopes for the future are good, and therefore pleasant. His mind is well stored too with subjects of contemplation. And he grieves and rejoices, more than any other, with himself; for the same thing is always painful, and the same thing always pleasant, and not one thing at one time and another at another; he has, so to speak, nothing to repent of.

The argument above, simplified, is just that the goods one wishes for one's friend are the same as one pursues for one's self.  

Therefore, since each of these characteristics belongs to the good man in relation to himself, and he is related to his friend as to himself (for his friend is another self), friendship too is thought to be one of these attributes, and those who have these attributes to be friends. Whether there is or is not friendship between a man and himself is a question we may dismiss for the present; there would seem to be friendship in so far as he is two or more, to judge from the afore-mentioned attributes of friendship, and from the fact that the extreme of friendship is likened to one's love for oneself.

Very often the above in bold is cited as the conclusion of Aristotle's enquiry into friendship: that the friend is another self (as was just demonstrated by the argument about wanting the same things for one's self that one does for one's friend). 

In this way, Aristotle has independently arrived at the conclusion that one ought to 'love one's friend as one's self'; and, since he began this chapter with the example of the friendship between neighbors, 'to love one's neighbor as one's self.' 

But the attributes named seem to belong even to the majority of men, poor creatures though they may be. Are we to say then that in so far as they are satisfied with themselves and think they are good, they share in these attributes? Certainly no one who is thoroughly bad and impious has these attributes, or even seems to do so. They hardly belong even to inferior people; for they are at variance with themselves, and have appetites for some things and rational desires for others. This is true, for instance, of incontinent people; for they choose, instead of the things they themselves think good, things that are pleasant but hurtful; while others again, through cowardice and laziness, shrink from doing what they think best for themselves.

So bad people aren't entirely capable of friendship, precisely because they don't even love themselves in the right way. How can you expect to wish the best for your 'other self' if you can't and don't do it for your own self? 

And those who have done many terrible deeds and are hated for their wickedness even shrink from life and destroy themselves. And wicked men seek for people with whom to spend their days, and shun themselves; for they remember many a grevious deed, and anticipate others like them, when they are by themselves, but when they are with others they forget. And having nothing lovable in them they have no feeling of love to themselves. Therefore also such men do not rejoice or grieve with themselves; for their soul is rent by faction, and one element in it by reason of its wickedness grieves when it abstains from certain acts, while the other part is pleased, and one draws them this way and the other that, as if they were pulling them in pieces. If a man cannot at the same time be pained and pleased, at all events after a short time he is pained because he was pleased, and he could have wished that these things had not been pleasant to him; for bad men are laden with repentance.

Is it true that bad men are laden with repentance, or that wicked men are? If one is thinking of drunkards as 'wicked men,' surely; they have hangovers, which feel like regret and sadness as well as pain. But it's not true of men like Blackbeard, who died defiantly rather than remorsefully; nor as far as we can tell of Aristotle's student Alexander, who put many men to the sword and thought it pleasant. Indeed, it isn't even true that Alexander would have thought himself wicked, although another pirate -- Blackbeard's ancient ancestor, perhaps -- once pointed Alexander's wickedness out to him.  Or so we are told on the authority of St. Augustine's City of God, in a chapter devoted to how alike kingdoms are to robbery!

Therefore the bad man does not seem to be amicably disposed even to himself, because there is nothing in him to love; so that if to be thus is the height of wretchedness, we should strain every nerve to avoid wickedness and should endeavour to be good; for so and only so can one be either friendly to oneself or a friend to another.

Leaving aside the question of whether all sorts of wickedness (or only some) produce repentance, the chapter closes with the proof that one ought to be good ourselves if only so we can be true friends to others -- or even ourselves. The proof is a reasonable one. If one ought to wish what is best for one's self, one ought to wish for virtue (since it is excellence in any category of human action, as established in Book I); and then, knowing how to properly wish it for one's self, one can properly wish it for one's friends. Thus one can be a true friend, as well as a good person.