311. Is God able to make a decree which sets up a most grave public necessity opposed to the observance of a law of nature?
(a) If there is question of absolute laws (see 303), this cannot be done, for God cannot deny Himself by making a disposition contrary to His Eternal Law. Example: We do not read that God ever sanctioned polyandry or marriage between parent and child, and it seems that He could never permit such things as lawful.
(b) If there is question of relative laws (see 303), the decree in question can be made by God; for the unbecomingness of that which is forbidden by a relative law passes away in the face of a great need. Example: Since God desired the propagation of the human race from one man and one woman, marriage between brothers and sisters was not against the Natural Law at the beginning. Since God desired the speedy multiplication of the chosen people after the patriarchal era, polygamy was not repugnant to nature among the Jews of that period.
312. Is God able to remove a natural obligation in a case of private necessity, that is, when the fulfillment would be harmful to an individual?
(a) Natural obligations that do not depend upon any free consent of the will given to them (see 304) cannot be removed except by a dispensation widely so-called and when their matter is contingent (as explained in 309-310). Examples: God could not dispense an individual from the duty of confessing Him in order to escape death, for the subject-matter of the law here is necessary. God, could dispense an individual from the obligation of not taking the property of another, for God is the principal owner of all things, including those possessed by others.
(b) Natural obligations that depend upon the act or deed of human beings consenting to obligation (see 304) can be removed. For since human beings cannot know all the circumstances existent, or all the conditions that will arise, it can happen that a thing agreed to or promised is only seemingly good, or will change from good to bad, so that while the promise or agreement made is in itself good and naturally obligatory, its fulfillment would work harm and evil, or be useless, or would prevent the accomplishment of a greater good. It is reasonable, therefore, that God should release from obligation here, thus changing the subject-matter of the law, so that it is no longer comprehended under the law (see 309-310). Example: Titus vows or swears that he will give a certain alms or make a certain pilgrimage; but, when the time for fulfillment arrives, his circumstances have so changed that it would not be advisable for him to keep the promise made. The Church, acting in the name of God, can declare that the subject-matter of this promise has become harmful and is not longer suitable, and hence that the obligation has ceased.
313. Human Authority and Modification of the Natural Law.
(a) Additions to the Natural Law may be made, not only by positive laws of God, but also by human laws of Church or State, through the introduction of that which Natural Law permits, or the determination or confirmation of that which Natural Law contains implicitly or explicitly. Examples: Division of property rights introduced by the law of nations; conditions for valid contracts determined by particular codes; the laws against theft and murder confirmed by definite penalties prescribed for those crimes.
(b) Subtractions from Natural Law cannot be made by any human authority, for God has not delegated His power of dispensing which He has as supreme owner of all things. Examples: No human authority could authorize a father to sacrifice his innocent son, nor permit a servant to carry away the effects that belong to his employer.
314. Apparent Cases of Dispensation from Natural Law made by Human Authority. (a) The Church frees from the obligation of vows, contracts and promissory oaths, from impediments to marriage, from espousals, etc. In so doing, however, she does not dispense from the Natural Law that vows, contracts, etc., should be fulfilled, but only declares in the name of God that the subject-matter of an obligation contracted by act of man’s will has become unsuitable for vow, contract, etc., and hence is no longer comprehended under the law.