§ 3. On the Immutability of Natural Law

127. PRINCIPLES. I. Natural law is subject to no change properly so called.

And indeed, a change of law is made either from within or from without: it is made from within when a law, previously reasonable and useful to the common good, afterward becomes unreasonable and harmful; but it is made from without when a legitimate Superior either derogates from it, or abrogates it totally, or finally dispenses subjects from it.

Now, natural law can be changed in none of these ways. 1. It cannot be changed from within; for by natural law those things are prescribed which necessarily flow from the essential relations of man, and those things are forbidden which are altogether opposed to the same essential relations. But the essential relations of man are immutable, as are the essences of things. Therefore.

2. It cannot be changed from without, not even by the authority of God the Supreme Legislator. For God cannot will the nature of things and not will those things which necessarily follow from it, because "he cannot deny himself" (2 Tim. 2:13). Therefore natural law, in those things which it prescribes or forbids, is absolutely immutable (Tongiorgi, Philos. mor., n. 179).

128. II. Natural law is subject to change improperly so called.

This change is made either by addition, namely when a new law is superadded to a law; or by subtraction of matter, namely when that which was the matter of a law ceases to be the object of the same law, on account of some supervening circumstance which changes it.

1. Natural law can improperly, or indirectly, be changed by addition. And in truth, many precepts have been added to it through positive laws, whether divine or human.

Thus, by natural right, the suretyship of a woman is valid, likewise marriage without witnesses is valid, etc.; and nevertheless by force of positive law they can be invalid. The reason is that these pertain only negatively to natural right, since natural law establishes nothing positively concerning them, but permits the same things to be done in this or that way.

2. It can also be changed indirectly by subtraction of matter, or rather by change of matter, namely in those precepts whose matter is subject to the dominion of God or of man.

Thus, for example, matter was subtracted from natural law when God commanded Abraham to sacrifice his son, or granted to the Hebrews the spoils of the Egyptians, or gave a precept of exterminating all the Canaanites. The same is done when God, or the Church in the name of God, dispenses in vows or oaths. For, when a just cause is present, God is judged to remit to man his debt as his proper lord, and thus to take away the vow by taking away the matter of the vow (Billuart, de Leg., Diss. 2, a. 4; Theol. Mechlin., de Leg., n. 38).

129. From these things you will gather: It is less rightly said that natural law ceases, has an exception, admits an excusing cause or interpretation, when, circumstances having changed, it no longer brings forth the same obligation (I, 174). Thus, for example, the obligation of fraternal correction does not cease, although it does not oblige here and now, if no fruit of amendment is hoped for; the obligation of returning a deposit and preserving a secret always urges, although, if grave harm to an innocent person or to the republic threatens from this, such and such a person becomes immune from this obligation. Cf. n. 216.