Objection 1. It would seem that man's happiness consists in pleasure. For since happiness is the last end, it is not desired for something else, but other things for it. But this answers to pleasure more than to anything else: "for it is absurd to ask anyone what is his motive in wishing to be pleased" (Ethic. x, 2). Therefore happiness consists principally in pleasure and delight.
Objection 2. Further, "the first cause goes more deeply into the effect than the second cause" (De Causis i). Now the causality of the end consists in its attracting the appetite. Therefore, seemingly that which moves most the appetite, answers to the notion of the last end. Now this is pleasure: and a sign of this is that delight so far absorbs man's will and reason, that it causes him to despise other goods. Therefore it seems that man's last end, which is happiness, consists principally in pleasure.
Objection 3. Further, since desire is for good, it seems that what all desire is best. But all desire delight; both wise and foolish, and even irrational creatures. Therefore delight is the best of all. Therefore happiness, which is the supreme good, consists in pleasure.
On the contrary, Boethius says (De Consol. iii): "Any one that chooses to look back on his past excesses, will perceive that pleasures had a sad ending: and if they can render a man happy, there is no reason why we should not say that the very beasts are happy too."
I answer that, Because bodily delights are more generally known, "the name of pleasure has been appropriated to them" (Ethic. vii, 13), although other delights excel them: and yet happiness does not consist in them. Because in every thing, that which pertains to its essence is distinct from its proper accident: thus in man it is one thing that he is a mortal rational animal, and another that he is a risible animal. We must therefore consider that every delight is a proper accident resulting from happiness, or from some part of happiness; since the reason that a man is delighted is that he has some fitting good, either in reality, or in hope, or at least in memory. Now a fitting good, if indeed it be the perfect good, is precisely man's happiness: and if it is imperfect, it is a share of happiness, either proximate, or remote, or at least apparent. Therefore it is evident that neither is delight, which results from the perfect good, the very essence of happiness, but something resulting therefrom as its proper accident.
But bodily pleasure cannot result from the perfect good even in that way. For it results from a good apprehended by sense, which is a power of the soul, which power makes use of the body. Now good pertaining to the body, and apprehended by sense, cannot be man's perfect good. For since the rational soul excels the capacity of corporeal matter, that part of the soul which is independent of a corporeal organ, has a certain infinity in regard to the body and those parts of the soul which are tied down to the body: just as immaterial things are in a way infinite as compared to material things, since a form is, after a fashion, contracted and bounded by matter, so that a form which is independent of matter is, in a way, infinite. Therefore sense, which is a power of the body, knows the singular, which is determinate through matter: whereas the intellect, which is a power independent of matter, knows the universal, which is abstracted from matter, and contains an infinite number of singulars. Consequently it is evident that good which is fitting to the body, and which causes bodily delight through being apprehended by sense, is not man's perfect good, but is quite a trifle as compared with the good of the soul. Hence it is written (Wisdom 7:9) that "all gold in comparison of her, is as a little sand." And therefore bodily pleasure is neither happiness itself, nor a proper accident of happiness.
Reply to Objection 1. It comes to the same whether we desire good, or desire delight, which is nothing else than the appetite's rest in good: thus it is owing to the same natural force that a weighty body is borne downwards and that it rests there. Consequently just as good is desired for itself, so delight is desired for itself and not for anything else, if the preposition "for" denote the final cause. But if it denote the formal or rather the motive cause, thus delight is desirable for something else, i.e. for the good, which is the object of that delight, and consequently is its principle, and gives it its form: for the reason that delight is desired is that it is rest in the thing desired.
Reply to Objection 2. The vehemence of desire for sensible delight arises from the fact that operations of the senses, through being the principles of our knowledge, are more perceptible. And so it is that sensible pleasures are desired by the majority.
Reply to Objection 3. All desire delight in the same way as they desire good: and yet they desire delight by reason of the good and not conversely, as stated above (ad 1). Consequently it does not follow that delight is the supreme and essential good, but that every delight results from some good, and that some delight results from that which is the essential and supreme good.
Source: Summa Theologica (New Advent)