Marsilius of Padua was an Italian scholar, trained in medicine who practiced a variety of professions. He was also an important 14th century political figure. His political treatise Defensor pacis is seen by some authorities as the most revolutionary political treatise written in the later Middle Ages.
Marsilius wrote Defensor pacis in 1324. This treatise was created in the context of a power struggle between Pope John XXII and Louis of Bavaria (or Ludwig of Bavaria), the elected candidate for Holy Roman Emperor. Louis' policies in the Italian peninsula, where the Empire had important territories, threatened papal territorial sovereignty. In 1323 Louis had sent an army to Italy to protect Milan against the powerful Kingdom of Naples. Naples, along with France, was a powerful ally of John XXII. John excommunicated Louis and demanded that he relinquish his claim to the imperial crown. Louis responded to John XXII with fresh provocations.
In Defensor pacis, Marsilius sought to demonstrate, by arguments from reason (in Dictio I of the text) and by argument from authority (in Dictio II) the independence of the Holy Roman Empire from the Papacy and the emptiness of the prerogatives alleged to have been usurped by the sovereign pontiffs. This demonstration was regarded as heretical.