Aristotle, Nicomachean Ethics
H. Rackham, Ed.

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. [6] The pleasure does not however perfect the activity in the same way as the object perceived and the sensory faculty, if good, perfect it; just as health and the physician are not in the same way the cause of being healthy. [7]

（It is clear that each of the senses is accompanied by pleasure, since we apply the term pleasant to sights and sounds¹; and it is also clear that the pleasure is greatest when the sensory faculty is both in the best condition and acting in relation to the best object; and given excellence in the perceived object and the percipient organ, there will always be pleasure when an object to cause it and a subject to feel it are both present.） [8]

But the pleasure perfects the activity, not as the fixed disposition does, by being already present in the agent, but as a supervening perfection, like the bloom of health in the young and vigorous.

So long therefore as both object thought of or perceived, and subject discerning or judging, are such as they should be,