The Complete Aristotle (eng.)

the lover who formerly
promised everything now performs nothing. Such incidents happen
when the lover loves the beloved for the sake of pleasure while the
beloved loves the lover for the sake of utility, and they do not
both possess the qualities expected of them. If these be the
objects of the friendship it is dissolved when they do not get the
things that formed the motives of their love; for each did not love
the other person himself but the qualities he had, and these were
not enduring; that is why the friendships also are transient. But
the love of characters, as has been said, endures because it is
self-dependent. Differences arise when what they get is something
different and not what they desire; for it is like getting nothing
at all when we do not get what we aim at; compare the story of the
person who made promises to a lyre-player, promising him the more,
the better he sang, but in the morning, when the other demanded the
fulfilment of his promises, said that he had given pleasure for
pleasure. Now if this had been what each wanted, all would have
been well; but if the one wanted enjoyment but the other gain, and
the one has what he wants while the other has not, the terms of the
association will not have been properly fulfilled; for what each in
fact wants is what he attends to, and it is for the sake of that
that that he will give what he has.
    But who is to fix the worth of the service; he who makes the
sacrifice or he who has got the advantage? At any rate the other
seems to leave it to him. This is what they say Protagoras used to
do; whenever he taught anything whatsoever, he bade the learner
assess the value of the knowledge, and accepted the amount so
fixed. But in such matters some men approve of the saying ‘let a
man have his fixed reward’. Those who get the money first and then
do none of the things they said they would, owing to the
extravagance of their promises, naturally find themselves the
objects of complaint; for they do not fulfil what they agreed to.
The sophists are perhaps compelled to do this because no one would
give money for the things they do know. These people then, if they
do not do what they have been paid for, are naturally made the
objects of complaint.
    But where there is no contract of service, those who give up
something for the sake of the other party cannot (as we have said)
be complained of (for that is the nature of the friendship of
virtue), and the return to them must be made on the basis of their
purpose (for it is purpose that is the characteristic thing in a
friend and in virtue). And so too, it seems, should one make a
return to those with whom one has studied philosophy; for their
worth cannot be measured against money, and they can get no honour
which will balance their services, but still it is perhaps enough,
as it is with the gods and with one’s parents, to give them what
one can.
    If the gift was not of this sort, but was made with a view to a
return, it is no doubt preferable that the return made should be
one that seems fair to both parties, but if this cannot be
achieved, it would seem not only necessary that the person who gets
the first service should fix the reward, but also just; for if the
other gets in return the equivalent of the advantage the
beneficiary has received, or the price lie would have paid for the
pleasure, he will have got what is fair as from the other.
    We see this happening too with things put up for sale, and in
some places there are laws providing that no actions shall arise
out of voluntary contracts, on the assumption that one should
settle with a person to whom one has given credit, in the spirit in
which one bargained with him. The law holds that it is more just
that the person to whom credit was given should fix the terms than
that the person who gave credit should do so. For most things are
not assessed at the same value by those who have them and those who
want them; each class values highly what is its own and what it is
offering; yet the return is made on the terms fixed by the
receiver. But no doubt the receiver should assess a thing not at
what it seems worth when he has it, but at what he assessed it at
before he had it.
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2
    A further problem is set by such questions as, whether one
should in all things give the preference to one’s father and obey
him, or whether when one is ill one should trust a doctor, and when
one has to elect a general should elect a man of military