Medieval political theory in Western Europe arose out of the controversy between Church and State over the question of the investiture of bishops by the secular powers. Since the clergy were virtually the only people who were literate and numerate, emperors, kings, dukes, and other rulers relied on their help in the administration of their domains. It was, therefore, important that, at the highest level, clerics should be not only able administrators but also sympathetic to the sovereign. To ensure this rulers took to refusing to recognize an unfavourable papal choice, and appointing a candidate of their own choice whom they invested with both spiritual and temporal power.
The papacy resisted this from the advent of Gregory VII (1073–85). Before his time the papacy had a tenuous claim on ecclesiastical supremacy even in spiritual matters and was fortunate to control the appointment of archbishops. Gregory claimed the primacy of the Pope, even in temporal matters. These included the deposition of rulers and absolving their subjects of allegiance. These claims were based on some texts of scripture (principally Matthew 16: 19, and Luke 22: 38), but above all on the eighth‐century forged document, The Donation of Constantine, which states that, on his conversion, the emperor Constantine handed over to Pope Sylvester I the imperial power in the West. These papal claims were reiterated throughout the following centuries and found their most extreme expression in the bull Unam Sanctam of Boniface VIII in 1302 which so provoked Philip IV of France that he had the Pope imprisoned.
Philosophers who supported the papal claims included Giles of Rome, John of Salisbury, and Aquinas. Those on the other side included Dante and—most extremely—Marsiglio of Padua. Giles of Rome (1247–1316) wrote two important political works while teaching theology in Paris between 1285 and 1292, De regimine principum and De potestate ecclesiastica. The first work was written for the future Philip IV. It was basically Aristotelian and Thomist. The second, which was papalist, was ironically the source on which Boniface VIII drew for the bull Unam Sanctam. The two can be reconciled only by saying that the first deals with the ruler merely in his temporal role whereas the second goes to the root of temporal and spiritual power. The first adds nothing to Aquinas. The second states the extreme papalist position based on Augustinian arguments. Giles maintained that all power came from God through the Church and in particular the Vicar of Christ, the Pope. He conferred temporal power on secular rulers and could, if necessary, withdraw their power and absolve their subjects of allegiance. Temporal power involved the power over life and this only God had, so it could only be conferred by God's representative, the Church, and, in particular the Vicar of Christ. However, in Giles's phrase the temporal power belonged to the Church non ad usum sed ad nusum, that is, it had it but would/could not use it itself.
John of Salisbury (c. 1115–80), an earlier papalist, who had been secretary to Thomas à Becket, took a less extreme line. In his Polycraticus he maintained that temporal power came from the hand of the Church. But he did not give it the power to depose rulers. That he left to the subjects. Like many medieval theorists, he supported tyrannicide. In his view a ruler became a tyrant when he transgressed the laws of natural morality or of natural justice (aequitas). He interpreted the Roman lawyer Ulpian's dictum, Quod principi placuit legis habet vigorem (What the ruler decides has the force of law) not in an absolutist sense, as if the ruler can legally do what he likes, but in the juristic sense that the ruler's legitimate legislation has force in virtue of the powers invested in him by the people. Thus his object was to restrict the scope of the temporal power rather than to enhance the power of the papacy. However, he was not as thoroughgoing a political theorist as Aquinas.