I answer that, We may speak of virtuous acts in two ways: first, under the aspect of virtuous; secondly, as such and such acts considered in their proper species. If then we speak of acts of virtue, considered as virtuous, thus all virtuous acts belong to the natural law. For it has been stated (A. 2) that to the natural law belongs everything to which a man is inclined according to his nature. Now each thing is inclined naturally to an operation that is suitable to it according to its form: thus fire is inclined to give heat. Wherefore, since the rational soul is the proper form of man, there is in every man a natural inclination to act according to reason: and this is to act according to virtue. Consequently, considered thus, all acts of virtue are prescribed by the natural law: since each one's reason naturally dictates to him to act virtuously. But if we speak of virtuous acts, considered in themselves, i.e. in their proper species, thus not all virtuous acts are prescribed by the natural law: for many things are done virtuously, to which nature does not incline at first; but which, through the inquiry of reason, have been found by men to be conducive to well-living.
Reply Obj. 1: Temperance is about the natural concupiscences of food, drink and sexual matters, which are indeed ordained to the natural common good, just as other matters of law are ordained to the moral common good.
Reply Obj. 2: By human nature we may mean either that which is proper to man—and in this sense all sins, as being against reason, are also against nature, as Damascene states (De Fide Orth. ii, 30): or we may mean that nature which is common to man and other animals; and in this sense, certain special sins are said to be against nature; thus contrary to sexual intercourse, which is natural to all animals, is unisexual lust, which has received the special name of the unnatural crime.
Reply Obj. 3: This argument considers acts in themselves. For it is owing to the various conditions of men, that certain acts are virtuous for some, as being proportionate and becoming to them, while they are vicious for others, as being out of proportion to them. ________________________
FOURTH ARTICLE [I-II, Q. 94, Art. 4]
Whether the Natural Law Is the Same in All Men?
Objection 1: It would seem that the natural law is not the same in all. For it is stated in the Decretals (Dist. i) that "the natural law is that which is contained in the Law and the Gospel." But this is not common to all men; because, as it is written (Rom. 10:16), "all do not obey the gospel." Therefore the natural law is not the same in all men.
Obj. 2: Further, "Things which are according to the law are said to be just," as stated in Ethic. v. But it is stated in the same book that nothing is so universally just as not to be subject to change in regard to some men. Therefore even the natural law is not the same in all men.
Obj. 3: Further, as stated above (AA. 2, 3), to the natural law belongs everything to which a man is inclined according to his nature. Now different men are naturally inclined to different things; some to the desire of pleasures, others to the desire of honors, and other men to other things. Therefore there is not one natural law for all.
On the contrary, Isidore says (Etym. v, 4): "The natural law is common to all nations."