Objection 1. It would seem that no pleasure is the greatest good. Because nothing generated is the greatest good: since generation cannot be the last end. But pleasure is a consequence of generation: for the fact that a thing takes pleasure is due to its being established in its own nature, as stated above (Question 31, Article 1). Therefore no pleasure is the greatest good.
Objection 2. Further, that which is the greatest good cannot be made better by addition. But pleasure is made better by addition; since pleasure together with virtue is better than pleasure without virtue. Therefore pleasure is not the greatest good.
Objection 3. Further, that which is the greatest good is universally good, as being good of itself: since that which is such of itself is prior to and greater than that which is such accidentally. But pleasure is not universally good, as stated above (Article 2). Therefore pleasure is not the greatest good.
On the contrary, Happiness is the greatest good: since it is the end of man's life. But Happiness is not without pleasure: for it is written (Psalm 15:11): "Thou shalt fill me with joy with Thy countenance; at Thy right hand are delights even to the end."
I answer that, Plato held neither with the Stoics, who asserted that all pleasures are evil, nor with the Epicureans, who maintained that all pleasures are good; but he said that some are good, and some evil; yet, so that no pleasure be the sovereign or greatest good. But, judging from his arguments, he fails in two points. First, because, from observing that sensible and bodily pleasure consists in a certain movement and "becoming," as is evident in satiety from eating and the like; he concluded that all pleasure arises from some "becoming" and movement: and from this, since "becoming" and movement are the acts of something imperfect, it would follow that pleasure is not of the nature of ultimate perfection. But this is seen to be evidently false as regards intellectual pleasures: because one takes pleasure, not only in the "becoming" of knowledge, for instance, when one learns or wonders, as stated above (32, 8, ad 2); but also in the act of contemplation, by making use of knowledge already acquired.
Secondly, because by greatest good he understood that which is the supreme good simply, i.e. the good as existing apart from, and unparticipated by, all else, in which sense God is the Supreme Good; whereas we are speaking of the greatest good in human things. Now the greatest good of everything is its last end. And the end, as stated above (1, 8; 2, 7) is twofold; namely, the thing itself, and the use of that thing; thus the miser's end is either money or the possession of money. Accordingly, man's last end may be said to be either God Who is the Supreme Good simply; or the enjoyment of God, which implies a certain pleasure in the last end. And in this sense a certain pleasure of man may be said to be the greatest among human goods.
Reply to Objection 1. Not every pleasure arises from a "becoming"; for some pleasures result from perfect operations, as stated above. Accordingly nothing prevents some pleasure being the greatest good, although every pleasure is not such.
Reply to Objection 2. This argument is true of the greatest good simply, by participation of which all things are good; wherefore no addition can make it better: whereas in regard to other goods, it is universally true that any good becomes better by the addition of another good. Moreover it might be said that pleasure is not something extraneous to the operation of virtue, but that it accompanies it, as stated in Ethic. i, 8.
Reply to Objection 3. That pleasure is the greatest good is due not to the mere fact that it is pleasure, but to the fact that it is perfect repose in the perfect good. Hence it does not follow that every pleasure is supremely good, or even good at all. Thus a certain science is supremely good, but not every science is.
Source: Summa Theologica (New Advent)