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.