Marsillius was born in Padua, an important Italian city, circa 1275. He initially trained in medicine, but was employed in various professions, including that of a soldier. He went to the University of Paris in 1311. The reputation which he had gained in what were then called the physical sciences soon caused him to be raised to the position of rector of the university for the first term of the year 1313.

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 and France were strong allies of John XXII. which was together with France the strongest ally of the papacy. John excommunicated Louis and called upon the Bavarian to 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.

Marsilius and John of Jandun, who has sometimes been credited as a co-author of Defensor pacis, left France for Louis' court in Bavaria. Louis IV admitted Marsilius and John to his circle. Others were also under his protection, including such philosophers such as Michael of Cesena and William of Ockham, who advocated a form of church and imperial separation. In 1326, Marsilius accompanied Louis IV to Italy, where he preached or circulated written attacks against the pope. The Lord of Milan Galeazzo I Visconti, suspected of conspiring with John, was deposed and Louis was crowned King of Italy in Milan in 1327.

In January 1328 Louis entered Rome and had himself crowned emperor by the aged senator Sciarra Colonna, called captain of the Roman people. Three months later Louis published a decree declaring "Jacque de Cahors" —Pope John XXII—deposed on grounds of heresy. He then installed the Franciscan Pietro Rainalducci as Nicholas V. Nicholas was deposed upon Louis departure from Rome in 1329.

In Bavaria, as imperial vicar, Marsilius persecuted the clergy who had remained faithful to John XXII. In recompense for his services, he was appointed archbishop of Milan, and John of Jandun obtained from Louis IV the bishopric of Ferrara.

Marsilius also composed a treatise De translatione [Romani] imperii, which some authorities consider is a rearrangement of a similar work by Landolfo Colonna called De jurisdictione imperatoris in causa matrimoniali. This work, and Marsilius' variation, sought to justify the exclusive jurisdiction of the emperor in matrimonial affairs: Louis of Bavaria had recently annulled the marriage of the son of the King of Bohemia.

Some authorities consider Defensor pacis one of the most important political and religious works of fourteenth-century Europe. In the Defensor minor, Marsilius completed and elaborated on different points in the doctrine laid down in the Defensor pacis. He dealt here with problems concerning ecclesiastical jurisdiction, penance, indulgences, crusades and pilgrimages, vows, excommunication, the general church council, marriage and divorce, and unity with the Greek Orthodox Church. In this work he even more clearly articulates imperial supremacy over the Church.

• The defender of peace (Cambridge University Press, 2005).
• Writings on the Empire: Defensor minor and De translatione imperii (Cambridge University Press, 1993).

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