Book 10
5
And since our activities are sharpened, prolonged and improved by their own pleasure, and impaired by the pleasures of other activities, it is clear that pleasures differ widely from each other. In fact alien pleasures have almost the same effect on the activities as their own painsi.e., the special pain accompanying a particular activity when it functions badly or in relation to a bad object.; since, when an activity causes pain, this pain destroys it, for instance, if a person finds writing or doing sums unpleasant and irksome; for he stops writing or doing sums, because the activity is painful. Activities then are affected in opposite ways by the pleasures and the pains that belong to them, that is to say, those that are intrinsically due to their exercise. Alien pleasures, as has been said, have very much the same effect as pain, for they destroy an activity, only not to the same degree.
