Questions 91 and 94 of the Summa Theologiae deal with the natural law. Peter Kreeft in the Shorter Summa, further shortens our approach by summarizing the two questions for us.
Question 91: Of the Various Kinds of Law
FIRST ARTICLE Whether There Is an Eternal Law?
I answer that, As stated above (Q. 90, A. I ad 2; AA. 3, 4), a law is nothing else but a dictate of practical reason emanating from the ruler who governs a perfect [complete] community. Now it is evident, granted that the world is ruled by Divine Providence, as was stated in the First Part (Q. 22, AA. I, 2), that the whole community of the universe is governed by Divine Reason. Wherefore the very Idea of the government of things in God the Ruler of the universe, has the nature of a law. And since the Divine Reason’s conception of things is not subject to time but is eternal, according to Proverbs 8:23, therefore it is that this kind of law must be called eternal…
Whether There Is in Us a Natural Law?
On the contrary, A gloss on Romans 2:14: When the Gentiles, who have not the [Mosaic] law, do by nature those things that are of the law, comments as follows: Although they have no written law, yet they have the natural law, whereby each one knows, and is conscious of, what is good and what is evil.
I answer that, As stated above (Q. 90, A. 1i ad 1i), law, being a rule and measure, can be in a person in two ways: in one way, as in him that rules and measures; in another way, as in that which is ruled and measured, since a thing is ruled and measured, in so far as it partakes of the rule or measure. Wherefore, since all things subject to Divine providence are ruled and measured by the eternal law, as was stated above (A. I); it is evident that all things partake somewhat of the eternal law, in so far as, namely, from its being imprinted on them, they derive their respective inclinations to their proper acts and ends.
Now among all others, the rational creature is subject to Divine providence in the most excellent way, in so far as it partakes of a share of providence, by being provident both for itself and for others. Wherefore it has a share of the Eternal Reason, whereby it has a natural inclination to its proper act and end: and this participation of the eternal law in the [very nature of the] rational creature is called the natural law.
Hence the Psalmist after saying (Psalms 4:6): Offer up the sacrifice of justice, as though someone asked what the works of justice are, adds: Many say, Who sheweth us good things? in answer to which question he says: The light of Thy countenance, O Lord, is signed upon us: thus implying that the light of natural reason, whereby we discern what is good and what is evil, which is the function of the natural law, is nothing else than an imprint on us of the Divine light.
It is therefore evident that the natural law is nothing else than the rational creature’s participation of the eternal law. [Thus the voice of conscience (natural reason judging good and evil) is the echo of the voice of God, and is therefore sacred and inviolable.that just as, in the speculative reason, from naturally known indemonstrable principles.]
Whether there Is a Human Law?
I answer that, As stated above (Q. 90, A. 1, ad 2) , a law is a dictate of the practical reason. Now it is to be observed that the same procedure takes place in the practical and in the speculative reason: for each proceeds from principles to conclusions, as stated above (ibid.).
Accordingly we conclude [Self-evident theoretical axioms like the law of non-contradiction. There are also self-evident practical axioms, both general ("Do good, avoid evil") and specific ("Be just"). These are "the precepts of the natural law", which, since it is in our nature, is also naturally known, just as first theoretical principles are.] we draw the conclusions of the various sciences, the knowledge of which is not imparted to us by nature, but acquired by the efforts of reason, so too it is from the precepts of the natural law, as from general and indemonstrable principles, that the human reason needs to proceed to the more particular determination of certain matters.
These particular determinations, devised by human reason, are called human laws. ["Human law" is "positive law", law posited (made) by man. Moral positivism reduces all moral law to this, denying the eternal law and the natural law. A philosopher could admit the natural law without admitting the eternal law, since one could know the effect without knowing the cause; therefore the argument between legal positivism and natural law does not depend only on whether or not God is admitted. St. Thomas would disagree with Dostoyevsky’s saying, "If God does not exist, everything is permissible."]
Whether There Was Any Need for a Divine Law? [The divine law is that part of the eternal law which God made known by special revelation.]
I answer that, Besides the natural and the human law it was necessary for the directing of human conduct to have a Divine law. And this for four reasons. First, because…man is ordained to an end of eternal happiness… Secondly … on account of the uncertainty of human judgment… Thirdly, because … man is not competent to judge of’ interior movements, that are hidden…Fourthly, because. . . human law cannot punish or forbid all evil deeds…
QUESTION 94 Of the Natural Law
Whether the Natural Law Can Be Changed?
I answer that, A change in the natural law may be understood in two ways. First, by way of addition. In this sense nothing hinders the natural law from being changed: since many things for the benefit of human life have been added over and above the natural law, both by the Divine law and by human laws. [E.g.,the Beatitudes and the "evangelical counsels" in the New Testament add significantly to the old law; or there is the obligation to vote in a modern democracy, but not in an ancient monarchy.]
Secondly, a change in the natural law may be understood by way of subtraction, so that what previously was according to the natural law, ceases to be so. In this sense, the natural law is altogether unchangeable in its first principles: but in its secondary principles, which, as we have said (A. 4), are certain detailed proximate conclusions drawn from the first principles, the natural law. . . may be changed in some particular cases of rare occurrence, through some special causes hindering the observance of such precepts, as stated above (A.4)….
Whether the Law of Nature Can Be Abolished from the Heart of Man?
On the contrary, Augustine says (Confessions ii): Thy law is written in the hearts of men, which iniquity itself effaces not. But the law which is written in men’s hearts is the natural law. Therefore the natural law cannot be blotted out.
I answer that, As stated above (AA. 4, 5), there belong to the natural law, first, certain most general precepts, that are known to all; and secondly, certain secondary and more detailed precepts, which are, as it were, conclusions following closely from first principles. As to those general principles, the natural law, in the abstract, can nowise be blotted out from men’s hearts.
But it [i.e., the knowledge of the moral law, not the "rectitude" or objective rightness of it.] is blotted out in the case of a particular action, in so far as reason is hindered from applying the general principle to a particular point of practice, on account of concupiscence or some other passion, as stated above (Q. 77, A. 2).
But as to the other, i.e., the secondary precepts, the natural law can be blotted out from the human heart … by vicious customs and corrupt habits, as among some men, theft, and even unnatural vices, as the Apostle states (Rom i), were not esteemed sinful. [The greatest harm done by vice is thus its blinding of the reason against even knowing good and evil (cf. John 7:17). Cf. the blithely self-confident justification of "unnatural vice" today.].