Aristotle, Nicomachean Ethics
H. Rackham, Ed.

and the other does not, it would not be a fair bargain; for it is the thing that a man happens to need that he sets his heart on, and only to get that is he ready to give what he does. 1. [5]

Which party's business is it to decide the amount of the return due? Should it be assessed by the one who proffers the initial service? Or rather by the one who receives¹ it, since the other by proffering it seems to leave the matter to him? This we are told was the practice of Protagoras²; when he gave lessons in any subject, he used to tell his pupil to estimate the value he set upon his knowledge, and accepted a fee of that amount. 1. [6] In such matters however some people prefer the principle of ‘the wage stated.’³ But people who take the money in advance, and then, having made extravagant professions, fail to perform what they undertook, naturally meet with complaints because they have not fulfilled their bargain. 1. [7] Perhaps however the sophists are bound to demand their fees in advance, since nobody would pay money for the knowledge which they possess.⁴ 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,⁵ no complaint arises, for a friendship based on virtue does not give rise to quarrels;