Objection 1. It would seem that not all the moral precepts belong to the law of nature. For it is written (Sirach 17:9): "Moreover He gave them instructions, and the law of life for an inheritance." But instruction is in contradistinction to the law of nature; since the law of nature is not learnt, but instilled by natural instinct. Therefore not all the moral precepts belong to the natural law.
Objection 2. Further, the Divine law is more perfect than human law. But human law adds certain things concerning good morals, to those that belong to the law of nature: as is evidenced by the fact that the natural law is the same in all men, while these moral institutions are various for various people. Much more reason therefore was there why the Divine law should add to the law of nature, ordinances pertaining to good morals.
Objection 3. Further, just as natural reason leads to good morals in certain matters, so does faith: hence it is written (Galatians 5:6) that faith "worketh by charity." But faith is not included in the law of nature; since that which is of faith is above nature. Therefore not all the moral precepts of the Divine law belong to the law of nature.
On the contrary, The Apostle says (Romans 2:14) that "the Gentiles, who have not the Law, do by nature those things that are of the Law": which must be understood of things pertaining to good morals. Therefore all the moral precepts of the Law belong to the law of nature.
I answer that, The moral precepts, distinct from the ceremonial and judicial precepts, are about things pertaining of their very nature to good morals. Now since human morals depend on their relation to reason, which is the proper principle of human acts, those morals are called good which accord with reason, and those are called bad which are discordant from reason. And as every judgment of speculative reason proceeds from the natural knowledge of first principles, so every judgment of practical reason proceeds from principles known naturally, as stated above (94, A2,4): from which principles one may proceed in various ways to judge of various matters. For some matters connected with human actions are so evident, that after very little consideration one is able at once to approve or disapprove of them by means of these general first principles: while some matters cannot be the subject of judgment without much consideration of the various circumstances, which all are not competent to do carefully, but only those who are wise: just as it is not possible for all to consider the particular conclusions of sciences, but only for those who are versed in philosophy: and lastly there are some matters of which man cannot judge unless he be helped by Divine instruction; such as the articles of faith.
It is therefore evident that since the moral precepts are about matters which concern good morals; and since good morals are those which are in accord with reason; and since also every judgment of human reason must needs by derived in some way from natural reason; it follows, of necessity, that all the moral precepts belong to the law of nature; but not all in the same way. For there are certain things which the natural reason of every man, of its own accord and at once, judges to be done or not to be done: e.g. "Honor thy father and thy mother," and "Thou shalt not kill, Thou shalt not steal": and these belong to the law of nature absolutely. And there are certain things which, after a more careful consideration, wise men deem obligatory. Such belong to the law of nature, yet so that they need to be inculcated, the wiser teaching the less wise: e.g. "Rise up before the hoary head, and honor the person of the aged man," and the like. And there are some things, to judge of which, human reason needs Divine instruction, whereby we are taught about the things of God: e.g. "Thou shalt not make to thyself a graven thing, nor the likeness of anything; Thou shalt not take the name of the Lord thy God in vain."
This suffices for the Replies to the Objections.
Source: Summa Theologica (New Advent)