This reasoning leads us up to a conclusion, which St. Thomas states thus (la 2ae, q. 94. art. 5):
"A conceivable way in which the Natural Law might be changed is the way of subtraction, that something should cease to be of the Natural Law that was of it before. Understanding change in this sense, the Natural Law is absolutely immutable in its first principles; but as to secondary precepts, which are certain detailed conclusions closely related to the first principles, the Natural Law is not so changed as that its dictate is not right in most cases steadily to abide by; it may however be changed in some particular case, and in rare instances, through some special causes impeding the observance of these secondary precepts."
The reason for this conclusion, more pregnant, it may be, than St.
Thomas himself discerned, is given briefly as follows (2a 2ae, q. 57, art. 2, ad 1):
"Human nature is changeable; and therefore what is natural to man may sometimes fail to hold good."
The precepts of Natural Law that fail to be applicable when human nature sinks below par, are only secondary precepts, and few even of them. Christianity brings human nature up to par, and _fulfils_ the Natural Law (St. Matt. v. 17), enjoining the observance of it in its integrity. This is the meaning of St. John Chrysostom's saying: "Of old not such an ample measure of virtue was proposed to us; ... but since the coming of Christ the way has been made much narrower." (_De Virginitate_, c. 44: cf. his 17th Homily on St. Matt. v. 37; indeed the doctrine is familiar in his pages.) Thus the prohibition of polygamy, being a secondary precept of the natural law, failed in its application in that age of lapsed humanity, when a woman was better one of many wives, protected by one husband, than exposed to promiscuous violence and lust. (Isaias iv. i.)
NOTE ON ROUSSEAU.
The ruler is the servant of the _good_ of the people, not of the _will_ of the people, except inasmuch as--
a. the _will_ of the people is an indication of their _good_, of which they are probable judges;
b. it is usually impossible to do _good_ to the people against their steady _will_.