Aristotle, Nicomachean Ethics
H. Rackham, Ed.

and the return made should be in proportion to the intention of the benefactor, since intention is the measure of a friend, and of virtue. This is the principle on which it would seem that payment ought to be made to those who have imparted instruction in philosophy; for the value of their service is not measurable in money, and no honor paid them could be an equivalent, but no doubt all that can be expected is that to them, as to the gods and to our parents, we should make such return as is in our Power. 1. [8]

When on the other hand the gift is not disinterested but made with a view to a recompense, it is no doubt the best thing that a return should be made such as both parties concur in thinking to be what is due. But failing such concurrence, it would seem to be not only inevitable but just that the amount of the return should be fixed by the party that received the initial service, since the donor will have recovered what the recipient really owes when he has been paid the value of the service to him, or the sum that he would have been willing to pay as the price of the pleasure. 1. [9] For in buying and selling also this seems to be the practice¹; and in some countries the law does not allow actions for the enforcement of voluntary covenants,² on the ground that when you have trusted a man you ought to conclude the transaction as you began it. For it is thought fairer for the price to be fixed by the person who received credit than by the one who gave credit.³ For as a rule those who have a thing value it differently from those who want to get it. For one's own possessions and gifts always seem to one worth a great deal; but nevertheless the repayment is actually determined by the valuation