Objection 1. It would seem that pleasure is not the measure or rule of moral good and evil. Because "that which is first in a genus is the measure of all the rest" (Metaph. x, 1). But pleasure is not the first thing in the moral genus, for it is preceded by love and desire. Therefore it is not the rule of goodness and malice in moral matters.
Objection 2. Further, a measure or rule should be uniform; hence that movement which is the most uniform, is the measure and rule of all movements (Metaph. x, 1). But pleasures are various and multiform: since some of them are good, and some evil. Therefore pleasure is not the measure and rule of morals.
Objection 3. Further, judgment of the effect from its cause is more certain than judgment of cause from effect. Now goodness or malice of operation is the cause of goodness or malice of pleasure: because "those pleasures are good which result from good operations, and those are evil which arise from evil operations," as stated in Ethic. x, 5. Therefore pleasures are not the rule and measure of moral goodness and malice.
On the contrary, Augustine, commenting on Psalm 7:10 "The searcher of hearts and reins is God," says: "The end of care and thought is the pleasure which each one aims at achieving." And the Philosopher says (Ethic. vii, 11) that "pleasure is the architect," i.e. the principal, "end [St. Thomas took "finis" as being the nominative, whereas it is the genitive--tou telous; and the Greek reads "He" (i.e. the political philosopher), "is the architect of the end."], in regard to which, we say absolutely that this is evil, and that, good."
I answer that, Moral goodness or malice depends chiefly on the will, as stated above (Question 20, Article 1); and it is chiefly from the end that we discern whether the will is good or evil. Now the end is taken to be that in which the will reposes: and the repose of the will and of every appetite in the good is pleasure. And therefore man is reckoned to be good or bad chiefly according to the pleasure of the human will; since that man is good and virtuous, who takes pleasure in the works of virtue; and that man evil, who takes pleasure in evil works.
On the other hand, pleasures of the sensitive appetite are not the rule of moral goodness and malice; since food is universally pleasurable to the sensitive appetite both of good and of evil men. But the will of the good man takes pleasure in them in accordance with reason, to which the will of the evil man gives no heed.
Reply to Objection 1. Love and desire precede pleasure in the order of generation. But pleasure precedes them in the order of the end, which serves a principle in actions; and it is by the principle, which is the rule and measure of such matters, that we form our judgment.
Reply to Objection 2. All pleasures are uniform in the point of their being the repose of the appetite in something good: and in this respect pleasure can be a rule or measure. Because that man is good, whose will rests in the true good: and that man evil, whose will rests in evil.
Reply to Objection 3. Since pleasure perfects operation as its end, as stated above (Question 33, Article 4); an operation cannot be perfectly good, unless there be also pleasure in good: because the goodness of a thing depends on its end. And thus, in a way, the goodness of the pleasure is the cause of goodness in the operation.
Source: Summa Theologica (New Advent)