Aristotle, Nicomachean Ethics
H. Rackham, Ed.

But things that are akin to things of different kinds must themselves differ in kind. [3] A still clearer proof may be drawn from the hindrance that activities receive from the pleasure derived from other activities. For instance, persons fond of the flute cannot give their attention to a philosophical discussion when they overhear someone playing the flute, because they enjoy music more than the activity in which they are engaged; therefore the pleasure afforded by the music of the flute impairs the activity of study. [4] The same thing occurs in other cases when a man tries to do two things at once; the pleasanter activity drives out the other, the more so if it is much more pleasant, until the other activity ceases altogether. Hence, when we enjoy something very much, we can hardly do anything else; and when we find a thing only mildly agreeable, we turn to some other occupation; for instance, people who eat sweets at the theater do so especially when the acting is bad. [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 pains¹; 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.