Since its proper pleasure heightens the exercise of a faculty, making it both more prolonged and better, while pleasure from another source spoils it, it is evident that there is a great difference between these two pleasures. Indeed, pleasure from another source has almost the same effect as pain from the activity itself. For the exercise of a faculty is spoilt by pain arising from it; as happens, for instance, when a man finds it disagreeable and painful to write or to calculate; for he stops writing in the one case and calculating in the other, since the exercise is painful. The exercise of a faculty, then, is affected in opposite ways by its proper pleasure and its proper pain; and by “proper” I mean that which is occasioned by the exercise itself. But pleasure from another source, we have already said, has almost the same effect as its proper pain; i.e. it interferes with the exercise of the faculty, though not to the same extent.