The Complete Aristotle (eng.)

however, mark off from the rest both the
friendship of kindred and that of comrades. Those of
fellow-citizens, fellow-tribesmen, fellow-voyagers, and the like
are more like mere friendships of association; for they seem to
rest on a sort of compact. With them we might class the friendship
of host and guest. The friendship of kinsmen itself, while it seems
to be of many kinds, appears to depend in every case on parental
friendship; for parents love their children as being a part of
themselves, and children their parents as being something
originating from them. Now (1) arents know their offspring better
than there children know that they are their children, and (2) the
originator feels his offspring to be his own more than the
offspring do their begetter; for the product belongs to the
producer (e.g. a tooth or hair or anything else to him whose it
is), but the producer does not belong to the product, or belongs in
a less degree. And (3) the length of time produces the same result;
parents love their children as soon as these are born, but children
love their parents only after time has elapsed and they have
acquired understanding or the power of discrimination by the
senses. From these considerations it is also plain why mothers love
more than fathers do. Parents, then, love their children as
themselves (for their issue are by virtue of their separate
existence a sort of other selves), while children love their
parents as being born of them, and brothers love each other as
being born of the same parents; for their identity with them makes
them identical with each other (which is the reason why people talk
of ‘the same blood’, ‘the same stock’, and so on). They are,
therefore, in a sense the same thing, though in separate
individuals. Two things that contribute greatly to friendship are a
common upbringing and similarity of age; for ‘two of an age take to
each other’, and people brought up together tend to be comrades;
whence the friendship of brothers is akin to that of comrades. And
cousins and other kinsmen are bound up together by derivation from
brothers, viz. by being derived from the same parents. They come to
be closer together or farther apart by virtue of the nearness or
distance of the original ancestor.
    The friendship of children to parents, and of men to gods, is a
relation to them as to something good and superior; for they have
conferred the greatest benefits, since they are the causes of their
being and of their nourishment, and of their education from their
birth; and this kind of friendship possesses pleasantness and
utility also, more than that of strangers, inasmuch as their life
is lived more in common. The friendship of brothers has the
characteristics found in that of comrades (and especially when
these are good), and in general between people who are like each
other, inasmuch as they belong more to each other and start with a
love for each other from their very birth, and inasmuch as those
born of the same parents and brought up together and similarly
educated are more akin in character; and the test of time has been
applied most fully and convincingly in their case.
    Between other kinsmen friendly relations are found in due
proportion. Between man and wife friendship seems to exist by
nature; for man is naturally inclined to form couples-even more
than to form cities, inasmuch as the household is earlier and more
necessary than the city, and reproduction is more common to man
with the animals. With the other animals the union extends only to
this point, but human beings live together not only for the sake of
reproduction but also for the various purposes of life; for from
the start the functions are divided, and those of man and woman are
different; so they help each other by throwing their peculiar gifts
into the common stock. It is for these reasons that both utility
and pleasure seem to be found in this kind of friendship. But this
friendship may be based also on virtue, if the parties are good;
for each has its own virtue and they will delight in the fact. And
children seem to be a bond of union (which is the reason why
childless people part more easily); for children are a good common
to both and what is common holds them together.
    How man and wife and in general friend and friend ought mutually
to behave seems to be the same question as how it is just for them
to behave; for a man does not seem to have the same duties to a
friend, a stranger, a comrade, and a