Dante, the Two Powers and the Heavens 

As Richard Kay put it, “the emperor can perform his natural function as the ‘caretaker of the world’ without the pope’s blessing, but he will act more effectively if he has the grace conferred by the pope’s blessing”. The relationship between the two universal powers therefore qualifies, in modern terms, as a relationship of reciprocal autonomy, although that of the emperor is a “reverent” autonomy, under the sole power of the One who alone “has no superior”.

Gratian, Distinctio prima, dictum ante canon 1: "Humanum genus duobus regitur, naturali videlicet iure et moribus."