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.