On the contrary, It is stated in the Decretals (Dist. xii, 5): "It is absurd, and a detestable shame, that we should suffer those traditions to be changed which we have received from the fathers of old."
I answer that, As stated above (A. 1), human law is rightly changed, in so far as such change is conducive to the common weal. But, to a certain extent, the mere change of law is of itself prejudicial to the common good: because custom avails much for the observance of laws, seeing that what is done contrary to general custom, even in slight matters, is looked upon as grave. Consequently, when a law is changed, the binding power of the law is diminished, in so far as custom is abolished. Wherefore human law should never be changed, unless, in some way or other, the common weal be compensated according to the extent of the harm done in this respect. Such compensation may arise either from some very great and every evident benefit conferred by the new enactment; or from the extreme urgency of the case, due to the fact that either the existing law is clearly unjust, or its observance extremely harmful. Wherefore the Jurist says [*Pandect. Justin. lib. i, ff., tit. 4, De Constit. Princip.] that "in establishing new laws, there should be evidence of the benefit to be derived, before departing from a law which has long been considered just."
Reply Obj. 1: Rules of art derive their force from reason alone: and therefore whenever something better occurs, the rule followed hitherto should be changed. But "laws derive very great force from custom," as the Philosopher states (Polit. ii, 5): consequently they should not be quickly changed.
Reply Obj. 2: This argument proves that laws ought to be changed: not in view of any improvement, but for the sake of a great benefit or in a case of great urgency, as stated above. This answer applies also to the Third Objection. ________________________
THIRD ARTICLE [I-II, Q. 97, Art. 3]
Whether Custom Can Obtain Force of Law?
Objection 1: It would seem that custom cannot obtain force of law, nor abolish a law. Because human law is derived from the natural law and from the Divine law, as stated above (Q. 93, A. 3; Q. 95, A. 2). But human custom cannot change either the law of nature or the Divine law. Therefore neither can it change human law.
Obj. 2: Further, many evils cannot make one good. But he who first acted against the law, did evil. Therefore by multiplying such acts, nothing good is the result. Now a law is something good; since it is a rule of human acts. Therefore law is not abolished by custom, so that the mere custom should obtain force of law.
Obj. 3: Further, the framing of laws belongs to those public men whose business it is to govern the community; wherefore private individuals cannot make laws. But custom grows by the acts of private individuals. Therefore custom cannot obtain force of law, so as to abolish the law.
On the contrary, Augustine says (Ep. ad Casulan. xxxvi): "The customs of God's people and the institutions of our ancestors are to be considered as laws. And those who throw contempt on the customs of the Church ought to be punished as those who disobey the law of God."