Aristotle's theory is, briefly, that every normal sense-perception or rational activity has its correspondent pleasure, and that the most perfect is the most pleasant: the most perfect in the case of any faculty being the exercise of the faculty in good condition on the best object. The pleasure follows the activity immediately, giving it a kind of finish, ``like the bloom of youth''. Pleasures vary in kind, as the activities that constitute life vary: the best pleasures are those of the philosophic life.

ME Book 2 Chapter 6 Section 2