He, then, who fails to do that for which he has already been paid, is rightly chargeable. But when there is no express agreement about the service rendered, (a) when one voluntarily helps another for that other’s sake, no accusation can arise, as we said: for this is the nature of friendship based on virtue. The return must here be regulated by the purpose of him who renders the first service; for it is purpose that makes both friend and virtue. The same rule would seem to apply also to the relations of a philosopher and his disciples; for desert cannot here be measured in money, and no honour that could be paid him would be an adequate return; but, nevertheless, as in our relations to gods and parents, the possible is accepted as sufficient. (b) If however, the first gift has been made, not in this spirit, but on the understanding that there shall be some return, the return should, if possible, be such as both deem proportionate to desert: but if this cannot be, it would seem to be not only necessary, but just, that the recipient of the first benefit should assess it; for whatever be the amount of the advantage he has received, or whatever he would have been willing to give for the pleasure, the other, in receiving the same amount, will receive as much as is due from him.