In the summer of 1324 a book began to circulate in Paris that was to send shock waves throughout Christendom. It was a long book, published anonymously under the simple title Defensor Pacis (The Defender of Peace), and its main thesis was that the chief impediment to civil peace was the papal claim to supreme jurisdiction in all things. The discussion of the struggle for sovereignty between the secular and ecclesiastical powers, between regnum and sacerdotium, which had dominated medieval political writing for centuries, had reached a high point with the flood of polemical literature occasioned by the battle for supremacy between Pope Boniface VIII and Philip the Fair of France around the turn of the fourteenth century. The Defensor Pacis, the climax of this long literary tradition, was written to refute the partisans of papal "plenitude of power," who claimed that the authority of the papacy was supreme...