Sometimes, again, friends quarrel when they find they are getting something different from what they want; for failing to get what you want is like getting nothing. This may be illustrated by the story of the harper: a man promised him that the better he played, the more he should receive; but when, as dawn drew near, the harper claimed the fulfilment of his promise, the other replied that he had already paid him pleasure 231 for pleasure. Now, if this was what both wished, there would be nothing more to say: but if the one wanted pleasure and the other profit, and the one has what he wants, while the other has not, the bargain will not be fairly carried out; for it is what a man happens to want that he sets his heart on, and consents for the sake of it to render this particular service.

But whose business is it to fix the value of the service? his who first gives, or rather his who first receives?⁠—for he who first gives seems to leave it to the other. This, they say, was the custom of Protagoras: when he had been giving lessons in any subject, he used to tell his pupil to estimate the value of the knowledge he had acquired, and so much he would take.