An interesting post on the OUP blog by Sophie Thérèse Ambler, courtesy my friend Bill Campbell:
What to do with a simple-minded ruler: a medieval solution
The thirteenth century saw the reigns of several rulers ill-equipped for the task of government, decried not as tyrants but incompetents. Sancho II of Portugal (1223–48), his critics said, let his kingdom fall to ruin on account of his “idleness,” “timidity of spirit,” and “simplicity”. The last term, simplex, could mean straightforward, but here it meant only simple-minded, foolish, stupid. The same term was used to describe the English king Henry III (1216–72), as well as John Balliol, the hapless king of Scotland (1292–96) appointed by England’s Edward I. As the elites of these kingdoms knew too well, it could happen on occasion that a man rose to office—whether he had been born to claim it, had won the right to hold it, or had found it thrust upon him—who did not have the intelligence to wield power.
Such a situation was dangerous, for subjects would suffer. In Portugal, it was claimed that Sancho’s inability to govern had allowed Church liberties to be attacked, women to be defiled, and the common folk to be oppressed. England’s Henry III had frittered away his resources, monies needed desperately to maintain his government; the result, it was claimed, was that Henry did not even have the cash to buy food and drink for his household and had turned to seizing victuals from his people, leaving them impoverished. The subjects of John Balliol had, perhaps, the most to fear from their king’s simplicity: John was incapable of standing up to Edward I, when a stand was needed urgently to defend his people from the bullying English king.
The people of Portugal, England, and Scotland knew of a potential solution to the problem of their simple-minded rulers: the rex inutilis theory (literally, “useless king”). This was a tenet of Church law that provided, when a bishop was too infirm to fulfill his duties, for the appointment of a coadjutor to exercise power on his behalf. The theory could be applied to lay rulers too, though it addressed here the problem of incompetence rather than infirmity.
It was the pope who held the power to pronounce a king rex inutilis. The papal court was like a medieval United Nations: its interests ranged from the making of peace between polities to the proper conduct of rulers, and the well-being of all those under the Church’s care. To this end, the pope had a mighty moral weapon in his arsenal: he could depose rulers and free subjects from their oaths of fealty or, as in the case of a rex inutilis, take effective power from his hands.
More at the link.