Papal Authority: Innocent III:  Letter to the prefect Acerbius and the nobles of Tuscany, 1198

Just as the founder of the universe established two great lights in the firmament of heaven, the greater light to rule the day, and the lesser light to rule the night, so too He set two great dignities in the firmament of the universal church..., the greater one to rule the day, that is, souls, and the lesser to rule the night, that is, bodies. These dignities are the papal authority and the royal power. Now just as the moon derives its light from the sun and is indeed lower than it in quantity and quality, in position and in power, so too the royal power derives the splendor of its dignity from the pontifical authority....