The Introduction gives a brief account of Bartolus’s life, explains the world of medieval law in which he worked, and then explains the political context of the northern and central Italian city republics for which he worked, and whose problems he sought to analyse. It explains that tyranny was Bartolus’s main preoccupation, even in the two treatises ostensibly concerned with other questions. It then presents the main arguments of his three political treatises and Bartolus’s main political theory in his academic legal commentaries, and describes the later influence of these treatises in European political theory. The Introduction also argues that Bartolus conceived of these three treatises as one composite treatment of tyranny.

In this treatise Bartolus applies the Aristotelian schema of constitutions to the city republics of his own day and argues that for the smallest such cities, such as Perugia, where Bartolus lived and worked, the most appropriate form of government is ‘government by the people’ or regimen ad populum. He argues that aristocracy is preferable in larger cities such as Venice and Florence, and then corrects Giles of Rome’s universal endorsement of monarchy as the best constitution by limiting it to much larger political organizations which hold sway over other peoples. He argues that where monarchy is appropriate at all, elective monarchy is superior to hereditary, and established by law for the Roman empire and for the church. Bartolus uses Roman constitutional development as presented in the Roman law to exemplify the different systems of rule brought into being by the growth in numbers and influence of a people. He casts his treatise in part as a lawyer’s version of the Aristotelian constitutional analysis made popular by the theologian Giles of Rome. The concept of the common good is central to Bartolus’s treatment.

In this treatise, Bartolus of Sassoferrato explores the phenomenon of factionalism in the fourteenth-century Italian city republics. He gives an account of the local nomenclature of Guelfs and Ghibellines, relating these labels historically to the papal and imperial camps in the contested region of northern Italy. He explains that, nowadays, such labels have only local relevance, not ideological significance. He then analyses the legality of joining such parties, concluding that if the reason for doing so is to uphold the common good, it is lawful.

Bartolus analyses the problem of tyranny according to biblical, Aristotelian, and legal authority. Starting from Pope Gregory the Great’s definition of the tyrant as one who rules without right in the commonwealth, Bartolus distinguishes between a tryant for want of just title, and a tyrant who possesses such just title but is tyrannical in his exercise of power. He is particularly interested in the validity or otherwise of legal transactions conducted by tyrants, and by those living under tyranny, and in how to prove by convincing legal means that a tyranny is or was in existence. The concept of fear, which invalidates certain legal agreements if proved, plays a major role in his argument here. He is especially interested in ‘veiled’ or covert tyrants, who have satisfied the legal formalities for legitimate government but are nonetheless tyrants. This leads him to explore the mechanics of popular election. Bartolus complicates the matter by noting that even legitimate governments need to behave in ways defined as tyrannical by Aristotle, and uses the concept of the common good as the ultimate criterion between legitimate and tyrannical rule.