298. According to their necessity for the primary or the secondary end of a natural inclination, the laws of nature are divided into primary and secondary. (a) The primary end of a natural inclination is the conservation of a natural good; and so it is a primary law of nature that man should take the food, drink, sleep and exercise necessary for life, and that he should avoid poison or other things that cause death. (b) The secondary end of a natural inclination is the betterment of a natural good, or its easier conservation; thus, it is a secondary law of nature that man should use those kinds of food or drink that promote his health, that he should be careful about his diet, practise moderation, etc.

299. Primary and secondary laws of nature are also explained as follows: (a) a primary law is one that expresses the principal purpose of a natural inclination (e.g., social good, that is, the begetting and rearing of children, is the primary law of the married state); (b) the secondary law is one that expresses a less important purpose of a natural inclination. For example, individual good (i.e., companionship, mutual assistance, the practice of virtue and freedom from temptation) is the secondary purpose to be promoted in the married state.

300. Precepts of the Natural Law may be divided also on account of the different relations they have to one another or to our knowledge.

(a) According to the priority they have among themselves, the laws of nature are divided into the first principle and the secondary principles. The first principle, which is general, which depends on no other, and which is the root of all the others, is: “Good must be done, evil omitted.” The secondary principles are particular, and they apply this general principle to the natural inclinations of man mentioned above, which reason indicates as ends of action—i.e., as goods to be sought.

(b) According to the priority they have with respect to our knowledge of them, the laws of nature are divided, first, into axiomatic precepts, which are evident and are granted by all (e.g., that good is to be done, that one should follow reason, that one should not do to others what one does not wish done to oneself etc.), and, secondly, into inferred precepts (e.g., that one should not steal from others, as one does not wish others to steal from oneself).

301. The inferred precepts are also of two kinds, namely, general and particular. (a) The general precepts are those that are deduced immediately from the axioms as universal conclusions (e.g., the commandments of the Decalogue, the principle that one should return what one borrowed). (b) The particular precepts are those that are deduced only remotely from the axioms as conclusions about cases in which many particular conditions and circumstances are involved (e.g., many conclusions about contracts, the conclusion that a loan is to be paid in some particular way, at this particular time, etc.).

302. According to the invariability or permanence of their subject-matter, the laws of nature are of two kinds, namely, necessary and contingent. (a) The necessary laws are those whose matter always bears the same relation of essential conformity to or difformity from reason. For example, the command, “Thou shalt not take the name of the Lord in vain,” is necessary, because God remains always worthy of honor, and there is no conceivable or possible case in which it could become useful to speak of Him with dishonor. (b) The contingent laws of nature are those whose matter generally, but not always, bears the same essential relation to right reason. For example, the command, “Thou shalt not kill,” is contingent, because, though man generally remains worthy of having his life respected by others, there are cases when it might be injurious to the common welfare, and hence to natural law, that an individual be permitted to live, as when he has committed and been convicted of a capital crime.

303. According to the manner in which they oblige, the laws of nature are twofold, namely, absolute and relative. (a) Absolute laws are those that oblige for every case and condition, because the matter with which they are concerned is intrinsically good or bad in every instance (e.g., the laws forbidding marriage between parent and child, the law against polyandry). (b) Relative laws of nature are those that oblige except in case of a most grave public necessity, because the matter with which they are concerned is generally and of its very nature becoming or unbecoming (e.g., the laws forbidding marriage between brother and sister, the law forbidding polygamy).

304. According to the manner in which the obligation is contracted, laws of nature are of two kinds, viz., those whose obligatory force depends entirely on the nature of things (e.g., the law that God must be honored), and those whose obligatory force depends upon, an act of the will of man freely undertaking an obligation, which the nature of things then demands that he fulfill (e.g., the laws that those who have made vows, oaths, contracts, etc., should live up to that which they have freely promised).

305. Properties.—Since the Natural Law is the reflection of God’s Eternal Law impressed on the rational nature of man, it has the following properties: (a) it is both declarative and imperative; being immanent in man, it declares to him his duty; being transcendent in its origin, it speaks with the voice of authority; (b) it is universal, or for all, for it declares the necessities of nature, which are the same in all men; (c) it is unchangeable, that is, it admits of neither abrogation, nor dispensation, nor emendatory interpretation, for the essences of things, on which it is based, do not change; (d) it is recognizable and indelible, that is, it cannot fail to be known and cannot be forgotten by mankind, for it is promulgated through the light of reason given to man.