Objection 1. It would seem that one pleasure cannot be contrary to another. Because the passions of the soul derive their species and contrariety from their objects. Now the object of pleasure is the good. Since therefore good is not contrary to good, but "good is contrary to evil, and evil to good," as stated in Praedic. viii; it seems that one pleasure is not contrary to another.
Objection 2. Further, to one thing there is one contrary, as is proved in Metaph. x, 4. But sadness is contrary to pleasure. Therefore pleasure is not contrary to pleasure.
Objection 3. Further, if one pleasure is contrary to another, this is only on account of the contrariety of the things which give pleasure. But this difference is material: whereas contrariety is a difference of form, as stated in Metaph. x, 4. Therefore there is no contrariety between one pleasure and another.
On the contrary, Things of the same genus that impede one another are contraries, as the Philosopher states (Phys. viii, 8). But some pleasures impede one another, as stated in Ethic. x, 5. Therefore some pleasures are contrary to one another.
I answer that, Pleasure, in the emotions of the soul, is likened to repose in natural bodies, as stated above (Question 23, Article 4). Now one repose is said to be contrary to another when they are in contrary termini; thus, "repose in a high place is contrary to repose in a low place" (Phys. v, 6). Wherefore it happens in the emotions of the soul that one pleasure is contrary to another.
Reply to Objection 1. This saying of the Philosopher is to be understood of good and evil as applied to virtues and vices: because one vice may be contrary to another vice, whereas no virtue can be contrary to another virtue. But in other things nothing prevents one good from being contrary to another, such as hot and cold, of which the former is good in relation to fire, the latter, in relation to water. And in this way one pleasure can be contrary to another. That this is impossible with regard to the good of virtue, is due to the fact that virtue's good depends on fittingness in relation to some one thing--i.e. the reason.
Reply to Objection 2. Pleasure, in the emotions of the soul, is likened to natural repose in bodies: because its object is something suitable and connatural, so to speak. But sadness is like a violent repose; because its object is disagreeable to the animal appetite, just as the place of violent repose is disagreeable to the natural appetite. Now natural repose is contrary both to violent repose of the same body, and to the natural repose of another, as stated in Phys. v, 6. Wherefore pleasure is contrary to both to another pleasure and to sadness.
Reply to Objection 3. The things in which we take pleasure, since they are the objects of pleasure, cause not only a material, but also a formal difference, if the formality of pleasurableness be different. Because difference in the formal object causes a specific difference in acts and passions, as stated above (23, 1,4; 30, 2).
Source: Summa Theologica (New Advent)