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 (Romans 10:16), "all do not obey the gospel." Therefore the natural law is not the same in all men.
Objection 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.
Objection 3. Further, as stated above (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."
I answer that, As stated above (2,3), to the natural law belongs those things to which a man is inclined naturally: and among these it is proper to man to be inclined to act according to reason. Now the process of reason is from the common to the proper, as stated in Phys. i. The speculative reason, however, is differently situated in this matter, from the practical reason. For, since the speculative reason is busied chiefly with the necessary things, which cannot be otherwise than they are, its proper conclusions, like the universal principles, contain the truth without fail. The practical reason, on the other hand, is busied with contingent matters, about which human actions are concerned: and consequently, although there is necessity in the general principles, the more we descend to matters of detail, the more frequently we encounter defects. Accordingly then in speculative matters truth is the same in all men, both as to principles and as to conclusions: although the truth is not known to all as regards the conclusions, but only as regards the principles which are called common notions. But in matters of action, truth or practical rectitude is not the same for all, as to matters of detail, but only as to the general principles: and where there is the same rectitude in matters of detail, it is not equally known to all.
It is therefore evident that, as regards the general principles whether of speculative or of practical reason, truth or rectitude is the same for all, and is equally known by all. As to the proper conclusions of the speculative reason, the truth is the same for all, but is not equally known to all: thus it is true for all that the three angles of a triangle are together equal to two right angles, although it is not known to all. But as to the proper conclusions of the practical reason, neither is the truth or rectitude the same for all, nor, where it is the same, is it equally known by all. Thus it is right and true for all to act according to reason: and from this principle it follows as a proper conclusion, that goods entrusted to another should be restored to their owner. Now this is true for the majority of cases: but it may happen in a particular case that it would be injurious, and therefore unreasonable, to restore goods held in trust; for instance, if they are claimed for the purpose of fighting against one's country. And this principle will be found to fail the more, according as we descend further into detail, e.g. if one were to say that goods held in trust should be restored with such and such a guarantee, or in such and such a way; because the greater the number of conditions added, the greater the number of ways in which the principle may fail, so that it be not right to restore or not to restore.
Consequently we must say that the natural law, as to general principles, is the same for all, both as to rectitude and as to knowledge. But as to certain matters of detail, which are conclusions, as it were, of those general principles, it is the same for all in the majority of cases, both as to rectitude and as to knowledge; and yet in some few cases it may fail, both as to rectitude, by reason of certain obstacles (just as natures subject to generation and corruption fail in some few cases on account of some obstacle), and as to knowledge, since in some the reason is perverted by passion, or evil habit, or an evil disposition of nature; thus formerly, theft, although it is expressly contrary to the natural law, was not considered wrong among the Germans, as Julius Caesar relates (De Bello Gall. vi).
Reply to Objection 1. The meaning of the sentence quoted is not that whatever is contained in the Law and the Gospel belongs to the natural law, since they contain many things that are above nature; but that whatever belongs to the natural law is fully contained in them. Wherefore Gratian, after saying that "the natural law is what is contained in the Law and the Gospel," adds at once, by way of example, "by which everyone is commanded to do to others as he would be done by."
Reply to Objection 2. The saying of the Philosopher is to be understood of things that are naturally just, not as general principles, but as conclusions drawn from them, having rectitude in the majority of cases, but failing in a few.
Reply to Objection 3. As, in man, reason rules and commands the other powers, so all the natural inclinations belonging to the other powers must needs be directed according to reason. Wherefore it is universally right for all men, that all their inclinations should be directed according to reason.
Source: Summa Theologica (New Advent)