Since, however, good has the nature of an end, and evil, the nature of a contrary, hence it is that all those things to which man has a natural inclination, are naturally apprehended by reason as being good, and consequently as objects of pursuit, and their contraries as evil, and objects of avoidance. Wherefore according to the order of natural inclinations, is the order of the precepts of the natural law. Because in man there is first of all an inclination to good in accordance with the nature which he has in common with all substances: inasmuch as every substance seeks the preservation of its own being, according to its nature: and by reason of this inclination, whatever is a means of preserving human life, and of warding off its obstacles, belongs to the natural law. Secondly, there is in man an inclination to things that pertain to him more specially, according to that nature which he has in common with other animals: and in virtue of this inclination, those things are said to belong to the natural law, "which nature has taught to all animals" [*Pandect. Just. I, tit. i], such as sexual intercourse, education of offspring and so forth. Thirdly, there is in man an inclination to good, according to the nature of his reason, which nature is proper to him: thus man has a natural inclination to know the truth about God, and to live in society: and in this respect, whatever pertains to this inclination belongs to the natural law; for instance, to shun ignorance, to avoid offending those among whom one has to live, and other such things regarding the above inclination.
Reply Obj. 1: All these precepts of the law of nature have the character of one natural law, inasmuch as they flow from one first precept.
Reply Obj. 2: All the inclinations of any parts whatsoever of human nature, e.g. of the concupiscible and irascible parts, in so far as they are ruled by reason, belong to the natural law, and are reduced to one first precept, as stated above: so that the precepts of the natural law are many in themselves, but are based on one common foundation.
Reply Obj. 3: Although reason is one in itself, yet it directs all things regarding man; so that whatever can be ruled by reason, is contained under the law of reason. ________________________
THIRD ARTICLE [I-II, Q. 94, Art. 3]
Whether All Acts of Virtue Are Prescribed by the Natural Law?
Objection 1: It would seem that not all acts of virtue are prescribed by the natural law. Because, as stated above (Q. 90, A. 2) it is essential to a law that it be ordained to the common good. But some acts of virtue are ordained to the private good of the individual, as is evident especially in regards to acts of temperance. Therefore not all acts of virtue are the subject of natural law.
Obj. 2: Further, every sin is opposed to some virtuous act. If therefore all acts of virtue are prescribed by the natural law, it seems to follow that all sins are against nature: whereas this applies to certain special sins.
Obj. 3: Further, those things which are according to nature are common to all. But acts of virtue are not common to all: since a thing is virtuous in one, and vicious in another. Therefore not all acts of virtue are prescribed by the natural law.
On the contrary, Damascene says (De Fide Orth. iii, 4) that "virtues are natural." Therefore virtuous acts also are a subject of the natural law.