Objection 1: It would seem that no pleasure is not natural. For pleasure is to the emotions of the soul what repose is to bodies. But the appetite of a natural body does not repose save in a connatural place. Neither, therefore, can the repose of the animal appetite, which is pleasure, be elsewhere than in something connatural. Therefore no pleasure is non-natural.
Objection 2: Further, what is against nature is violent. But "whatever is violent causes grief" (Metaph. v, 5). Therefore nothing which is unnatural can give pleasure.
Objection 3: Further, the fact of being established in one's own nature, if perceived, gives rise to pleasure, as is evident from the Philosopher's definition quoted above (A[1]). But it is natural to every thing to be established in its nature; because natural movement tends to a natural end. Therefore every pleasure is natural.
On the contrary, The Philosopher says (Ethic. vii, 5,6) that some things are pleasant "not from nature but from disease."
I answer that, We speak of that as being natural, which is in accord with nature, as stated in Phys. ii, 1. Now, in man, nature can be taken in two ways. First, inasmuch as intellect and reason is the principal part of man's nature, since in respect thereof he has his own specific nature. And in this sense, those pleasures may be called natural to man, which are derived from things pertaining to man in respect of his reason: for instance, it is natural to man to take pleasure in contemplating the truth and in doing works of virtue. Secondly, nature in man may be taken as contrasted with reason, and as denoting that which is common to man and other animals, especially that part of man which does not obey reason. And in this sense, that which pertains to the preservation of the body, either as regards the individual, as food, drink, sleep, and the like, or as regards the species, as sexual intercourse, are said to afford man natural pleasure. Under each kind of pleasures, we find some that are "not natural" speaking absolutely, and yet "connatural" in some respect. For it happens in an individual that some one of the natural principles of the species is corrupted, so that something which is contrary to the specific nature, becomes accidentally natural to this individual: thus it is natural to this hot water to give heat. Consequently it happens that something which is not natural to man, either in regard to reason, or in regard to the preservation of the body, becomes connatural to this individual man, on account of there being some corruption of nature in him. And this corruption may be either on the part of the body---from some ailment; thus to a man suffering from fever, sweet things seem bitter, and vice versa---or from an evil temperament; thus some take pleasure in eating earth and coals and the like; or on the part of the soul; thus from custom some take pleasure in cannibalism or in the unnatural intercourse of man and beast, or other such things, which are not in accord with human nature.
This suffices for the answers to the objections.
Whether one pleasure can be contrary to another?
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 (Q[23], A[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 (Q[23], AA[1],4; Q[30], A[2]).