Aristotle, Nicomachean Ethics
H. Rackham, Ed.

[14] If then the function of man is the active exercise of the soul's faculties¹ in conformity with rational principle, or at all events not in dissociation from rational principle, and if we acknowledge the function of an individual and of a good individual of the same class （for instance, a harper and a good harper, and so generally with all classes） to be generically the same, the qualification of the latter's superiority in excellence being added to the function in his case （I mean that if the function of a harper is to play the harp, that of a good harper is to play the harp well）: if this is so, and if we declare that the function of man is a certain form of life, and define that form of life as the exercise of the soul's faculties and activities in association with rational principle,