The natural law is a participation of the eternal law in the rational creature. Man is both ruler and subject of the natural law in the nuanced sense described earlier in the commentary on Question 91. Question 94 brings us to a deeper understanding of the natural law.

The legislative process of the ruler: We do not "make up" the precepts of natural law. These are not devised by human reason but passed on by the Supreme Ruler who has imprinted them on our nature. Our "legislative process" is simply that the precepts imprinted on our nature are issued to us as self-evident principles of the practical reason. Since these precepts are not of human origin they are shared by each creature which has a human nature (I II 94.4) and cannot be changed (I II 94.5).

Scope of natural law: "To natural law belongs everything to which man is inclined according to his nature." (I II 94.3) Now, man has an inclination particular to his rational nature, an inclination to good, but man's nature also contains the vegetative and animal natures. Each type of nature has inclinations according to that nature. Plants and animals follow these inclinations "without thinking". They passively participate in the eternal law to which they obey by necessity.

With the nature comes the inclination. Man has the same inclinations as plants and animals. Thus, man shares with the plant the inclination to grow and preserve his life and shares with the animal the inclination to reproduce and educate offspring.

Unlike substances of the lower levels, man can act against these natural inclinations by commiting suicide (which a plant would not do) or by having sexual intercourse without the possibility of reproduction (which an animal would not do).

In responding to each of his inclinations associated with the lower levels, for instance the biological laws, man encounters the first precept of the natural law (good is to be done and evil avoided) along with a secondary precept in accordance with the particular inclination. By simultaneously obeying the first and secondary precepts man converges the particular inclinations of the lower order with his peculiar inclination to good, determining how to use his body to provide for himself and others.

It is by coming under the audience of reason that goods which are not peculiar to man come to be constituents of the human good. Sex is a human good not just as such but insofar as it is engaged in consciously, purposively and responsibly. That is how it becomes a human evil too. There is no way in which humans can engage in sexual activity other than deliberately, which is why the animal part of our nature is always a part, never autonomous.

Ralph McInerny, Ethica Thomistica, 1996, Chapter 3.

Man also discovers, while determining how to use his body, that when he acts contrary to his plant and animal natures, rather than liberating him from these natures, he is acting against his very nature (that nature which he has in common with plants and animals and that nature which directs him to his end and the acts that lead to that end).

Denying that the natural inclinations below the rational level belong to the natural law and therefore have moral content, though popular in our times, separates the soul from the body and discards a human nature which integrates the lower natures. Human nature is assumed to be simply rational, waiting to be "defined" by "freedom".

Precepts of the natural law are the same for all who have a human nature, so why do people disagree about what is right and wrong? Good and bad? Thomas points out that, although the most basic precepts of the practical reason are always true and correct and can never be blotted out, the reliability, the "truthfulness", of conclusions from these first principles can be defective (I II94.4) or even blotted out altogether (I II 94.6) either due to:

knowledge: a misconception by the moral agent of the good. The reason can become perverted through passion (I II 77.2) or "evil persuasion". At a later point, regarding natural inclinations, Thomas writes: "Man can use reason to devise means of satisfying his lusts and passions," his natural inclinations emanating from the lower orders, which "animals are unable to do". (I II 95.1)

or

rectitude: the integrity of the determination of practical reason deteriorates as its conclusions move further from the first principles.

This raises the question of the undeniable experience of cultural conditioning of moral norms. However, differences between cultures does not disprove natural law but underlines the "noise in the system" which develops over time. Human nature comes first, designed by God when he created the first human being. Cultures came later. To use culturally opposed moral norms to disprove nature law can only be logically successful by denying that God designed anything in particular when he designed the human being.

Have we abandoned Aristotle in our attempt to elucidate Thomas' theory of natural law? Not entirely. Aristotle is content to present topics "in outline" (Nicomachean Ethics i 2) whereas Thomas does not mind putting content onto Aristotle's outline. Aristotle does not refer to natural law but to natural justice and only in these few words: "That which everywhere has the same force and does not exist by people's thinking this or that." (Nichomachean Ethics v 7) thus leaving Thomas ample room to provide content.