Aristotle, Nicomachean Ethics
H. Rackham, Ed.

Again, inasmuch as each of the senses acts in relation to its object, and acts perfectly when it is in good condition and directed to the finest of the and objects that belong to it （for this seems to be the best description of perfect activity, it being assumed to make no difference whether it be the sense itself that acts or the organ in which the sense resides）, it follows that the activity of any of the senses is at its best when the sense-organ being in the best condition is directed to the best of its objects; and this activity will be the most perfect and the pleasantest. For each sense has a corresponding pleasure, as also have thought and speculation, and its activity is pleasantest when it is most perfect, and most perfect when the organ is in good condition and when it is directed to the most excellent of its objects; and the pleasure perfects the activity.