Duo sunt
Duo sunt is a letter written in 494 by Pope Gelasius I to emperor Anastasius I.[1]
Dualistic principle of Church and State
This letter established the dualistic principle that would underlie all Western European political thought for almost a millennium. In the letter, Gelasius expressed a distinction between "two powers", which he called the "holy authority of bishops" (auctoritas sacrata pontificum) and the "royal power" (regalis potestas).
Potestas and auctoritas
These two powers, auctoritas lending justification to potestas, and potestas providing the executive strength for auctoritas were, he said, to be considered independent in their own spheres of operation, yet expected to work together in harmony.
Sovereign immunity
The letter played a significant role in the development of the legal doctrine of sovereign immunity, in that it gave political protections to the papacy and the monarchy, who promised not to violate each other's respective jurisdictions. This doctrine remains in force in international politics, even though most absolute monarchies have been replaced by constitutional monarchies or republics.