We give something additional even to those who teach the meanest trades, if their efforts appear to be extraordinary; we bestow a gratuity upon pilots, upon workmen who deal with the commonest materials and hire themselves out by the day. In the noblest arts, however, those which either preserve or beautify our lives, a man would be ungrateful who thinks he owes the artist no more than he bargained for. Besides this, the teaching of such learning as we have spoken of blends mind with mind; now when this takes place, both in the case of the physician and of the teacher the price of his work is paid, but that of his mind remains owing.