Leo bases his philosophy of society on
Aquinas's theory of natural law. Leo had promoted the study of scholastic philosophy in his earlier encyclical
Aeterni Patris in 1879. He rejects the claim that the Church is opposed to the rightful aims of the civil government. Man's natural instinct moves him to live in civil society, for he cannot, if dwelling apart, provide himself with the necessary requirements of life, nor procure the means of developing his mental and moral faculties. Hence it is divinely ordained that he should lead his life, be it family, social, or civil, with his fellow-men, amongst whom alone his several wants can be adequately supplied. But as no society can hold together unless someone be over all, directing all to strive earnestly for the common good, every civilized community must have a ruling authority, and this authority, no less than society itself, has its source in nature, and has consequently God for its author. Therefore, all authority ultimately derives from God. Leo alienated both the monarchists and the followers of
Lamennais in declining to specify what form government should take. "The right to rule is not necessarily bound up with any special mode of government. It may take this or that form, provided only that it be of a nature to ensure the general welfare. But whatever be the nature of the government, rulers must ever bear in mind that God is the paramount ruler of the world and must set Him before themselves as their exemplar and law in the administration of the State." "Nature and reason, which command every individual devoutly to worship God in holiness, because we belong to Him and must return to Him since from Him we came, bind also the civil community by a like law. ... So, too, is it a sin in the State not to have care for religion, as a something beyond its scope, or as of no practical benefit ... All who rule, therefore, should hold in honour the holy name of God, and one of their chief duties must be to favour religion, to protect it, to shield it under the credit and sanction of the laws, and neither to organize nor enact any measures that may compromise its safety." Leo made it quite clear that he opposed liberty of thought and liberty of the press: "So, too, the liberty of thinking, and of publishing, whatsoever each one likes, without any hindrance, is not in itself an advantage over which society can wisely rejoice. On the contrary, it is the fountain-head and origin of many evils." ==Relation between both powers ==