While still practising medicine he entered into relations with another master of Paris, the philosopher John of Jandun, who collaborated with him in the composition of the famous Defensor pacis (1324), one of the most extraordinary political and religious works which appeared during the 14th century.
The Defensor pacis, as its name implies, is a work intended to restore peace, as the most indispensable benefit of human society.
Deposed by the head of the Empire (April 18), and a mendicant friar, Pietro de Corbara, raised by an imperial decree to the throne of St Peter (as Nicholas V.) after a sham of a popular election (May 12), all this was merely the application of principles laid down in the Defensor pacis.
But, above all, in an unpublished work preserved at Oxford, the Defensor minor, Marsilius completed and elaborated in a curious manner certain points in the doctrine laid down in the Defensor pacis.
But the scandal provoked by his Defensor pacis, condemned by the court of Avignon in 1326, lasted much longer.