Aristotle, Nicomachean Ethics
H. Rackham, Ed.

it is profitable to receive one. [9] One ought therefore, if one can, to return the equivalent of services received, and to do so willingly; for one ought not to make a man one's friend if one is unwilling to return his favors. Recognizing therefore that one has made a mistake at the beginning and accepted a service from a wrong person—that is, a person who was not a friend, and was not acting disinterestedly¹ —one should accordingly end the transaction as if one had accepted the service on stated terms. Also, one would agree² to repay a service if able to do so （and if one were not able, the giver on his side too would not have expected repayment）; hence, if possible, one ought to make a return. But one ought to consider at the beginning from whom one is receiving the service, and on what terms, so that one may accept it on those terms or else decline it. [10]

Dispute may arise however as to the value of the service rendered. Is it to be measured by the benefit to the recipient, and the return made on that basis, or by the cost to the doer? The recipient will say that what he received was only a trifle to his benefactor, or that he could have got it from someone else: he beats down the value. The other on the contrary will protest that it was the most valuable thing he had to give, or that it could not have been obtained from anybody else, or that it was bestowed at a time of danger or in some similar emergency. [11] Perhaps then we may say that, when the friendship is one of utility, the measure of the service should be its value to the recipient, since it is he who wants it, and the other comes to his aid in the expectation of an equivalent return; therefore the degree of assistance rendered has been the amount to which the recipient has benefited,