Aristotle, Nicomachean Ethics
H. Rackham, Ed.

[7] Perhaps however the sophists are bound to demand their fees in advance, since nobody would pay money for the knowledge which they possess.¹ Persons paid in advance then naturally meet with complaints if they do not perform the service for which they have taken the pay.

But in cases where no agreement is come to as to the value of the service, if it is proffered for the recipient's own sake, as has been said above,² no complaint arises, for a friendship based on virtue does not give rise to quarrels; 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.