Friend as Second Self
Aristotle
Nicomachean Ethics

The author posits that because existence is inherently good and pleasant to the virtuous person, the existence of a friend—who is a 'second self'—is equally choice-worthy.

...perceives that he sees; and he that hears perceives that he hears; and he that walks perceives that he walks; and in all the other instances in like manner there is a faculty which reflects upon and perceives the fact that we are working, so that we can perceive that we perceive and intellectually know that we intellectually know: but to perceive that we perceive or that we intellectually know is to perceive that we exist, since existence was defined to be perceiving or intellectually knowing.
Now to perceive that one lives is a thing pleasant in itself, life being a thing naturally good, and the perceiving of the presence in ourselves of things naturally good being pleasant. Therefore the fact of living is choice-worthy, and to the good specially so since existence is good and pleasant to them: for they receive pleasure from the internal consciousness of that which in itself is good. But the good man is to his friend as to himself, friend being but a name for a second Self; therefore as his own existence is choice-worthy to each so too, or similarly at least, is his friend’s existence.
But the ground of one’s own existence being choice-worthy is the perceiving of one’s self being good, any such perception being in itself pleasant. Therefore one ought to be thoroughly conscious of one’s friend’s existence, which will result from living with him, that is sharing in his words and thoughts: for this is the meaning of the term as applied to the human species, not mere feeding together as in the case of brutes. If then to the man in a high state of happiness existence is in itself...
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Unity Among the Good
Aristotle
Nicomachean Ethics

The passage describes 'Unity of Sentiment' as a social friendship found among the good who share steady goals, contrasted with the bad who are divided by selfishness and jealousy.

...ians, or appointing Pittacus ruler (that is to say, supposing he himself was willing). But when each wishes himself to be in power (as the brothers in the Phœnissæ), they quarrel and form parties: for, plainly, Unity of Sentiment does not merely imply that each entertains the same idea be it what it may, but that they do so in respect of the same object, as when both the populace and the sensible men of a State desire that the best men should be in office, because then all attain their object.
Thus Unity of Sentiment is plainly a social Friendship, as it is also said to be: since it has for its object-matter things expedient and relating to life. And this Unity exists among the good: for they have it towards themselves and towards one another, being, if I may be allowed the expression, in the same position: I mean, the wishes of such men are steady and do not ebb and flow like the Euripus, and they wish what is just and expedient and aim at these things in common. The bad, on the contrary, can as little have Unity of Sentiment as they can be real friends, except to a very slight extent, desiring as they do unfair advantage in…
The result is that they quarrel while they are for keeping one another to work but are not willing to perform their just share. Chapter VII. Benefactors are commonly held to have more Friendship for the objects of their kindness than these for them: and the fact is made a subject of discussion and enquiry, as being contrary to reasonable expectation. The account of the matter which satisfies most persons is that the one are debtors and the others creditors: and therefore that, as in the case...
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Friendship's Common Goods
Aristotle
Nicomachean Ethics

The author argues that Friendship and Justice are co-extensive and share the same object-matter because both are rooted in the principle of communion.

...rse if they are equally fit objects of Friendship they are perhaps entitled to claim this, but if they have nothing of the kind it is ridiculous. Perhaps, moreover, the contrary does not aim at its contrary for its own sake but incidentally: the mean is really what is grasped at; it being good for the dry, for instance, not to become wet but to attain the mean, and so of the hot, etc. However, let us drop these questions, because they are in fact somewhat foreign to our purpose. Chapter IX.
It seems too, as was stated at the commencement, that Friendship and Justice have the same object-matter, and subsist between the same persons: I mean that in every Communion there is thought to be some principle of Justice and also some Friendship: men address as friends, for instance, those who are their comrades by sea, or in war, and in like manner also those who are brought into Communion with them in other ways: and the Friendship, because also the Justice, is co-extensive with the Communion, This justifies the common proverb, “the goods of friends are common,” since Friendship rests upon Communion.
Now brothers and intimate companions have all in common, but other people have their property separate, and some have more in common and others less, because the Friendships likewise differ in degree. So too do the various principles of Justice involved, not being the same between parents and children as between brothers, nor between companions as between fellow-citizens merely, and so on of all the other conceivable Friendships. Different also are the principles of Injustice as regards these...
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Friendship's Distance
Aristotle
Nicomachean Ethics

The text observes that physical distance and lack of intercourse can erode friendship, as human nature naturally avoids the painful and seeks the pleasurable.

...result and termed friends from their resemblance to these last. Chapter VI. Further; just as in respect of the different virtues some men are termed good in respect of a certain inward state, others in respect of acts of working, so is it in respect of Friendship: I mean, they who live together take pleasure in, and impart good to, one another: but they who are asleep or are locally separated do not perform acts, but only are in such a state as to act in a friendly way if they acted at all:
Distance has in itself no direct effect upon Friendship, but only prevents the acting it out: yet, if the absence be protracted, it is thought to cause a forgetfulness even of the Friendship: and hence it has been said, “many and many a Friendship doth want of intercourse destroy.” Accordingly, neither the old nor the morose appear to be calculated for Friendship, because the pleasurableness in them is small, and no one can spend his days in company with that which is positively painful or even not pleasurable; since to avoid the painful and aim at the pleasurable is one of the most obvious tendencies of human nature.
They who get on with one another very fairly, but are not in habits of intimacy, are rather like people having kindly feelings towards one another than friends; nothing being so characteristic of friends as the living with one another, because the necessitous desire assistance, and the happy companionship, they being the last persons in the world for solitary existence: but people cannot spend their time together unless they are mutually pleasurable and take pleasure in the same objects, a qual...
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Justice's Relational Scale
Aristotle
Nicomachean Ethics

The author argues that the obligations of justice and the severity of injustice increase in proportion to the closeness of the friendship or relationship involved.

...s by sea, or in war, and in like manner also those who are brought into Communion with them in other ways: and the Friendship, because also the Justice, is co-extensive with the Communion, This justifies the common proverb, “the goods of friends are common,” since Friendship rests upon Communion. Now brothers and intimate companions have all in common, but other people have their property separate, and some have more in common and others less, because the Friendships likewise differ in degree.
So too do the various principles of Justice involved, not being the same between parents and children as between brothers, nor between companions as between fellow-citizens merely, and so on of all the other conceivable Friendships. Different also are the principles of Injustice as regards these different grades, and the acts become intensified by being done to friends; for instance, it is worse to rob your companion than one who is merely a fellow-citizen; to refuse help to a brother than to a stranger; and to strike your father than any one else. So then the Justice naturally increases with the degree of Friendship, as being between the same parties and of equal extent.
All cases of Communion are parts, so to say, of the great Social one, since in them men associate with a view to some advantage and to procure some of those things which are needful for life; and the great Social Communion is thought originally to have been associated and to continue for the sake of some advantage: this being the point at which legislators aim, affirming that to be just which is generally expedient. All the other cases of Communion aim at advantage in particular points; the c...
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