Baldus' political thought possesses a fundamental structure which underlies and informs his whole treatment of public law: the acceptance that universally sovereign authorities, in the form of the emperor and the pope, coexist with territorially sovereign entities, that is independent city-republics and kingdoms. The explanation and elaboration of the origin, nature, operation and interrelation of these two forms of sovereignty form the core of his political ideas. It could be objected that adherence to the notion of universal sovereignty precludes the recognition of a genuinely territorial kind, on the grounds that the so-called territorially sovereign power would thus have a superior which would be a contradiction in terms: that the two ideas are in short mutually exclusive. This, however, would be to apply a modern view of sovereignty. When seen from an historical perspective sovereignty is a term whose meaning has changed through time: Bodin, for instance, considered that no jurist or political philosopher writing previously had been able to define the concept (and thus really understand it); yet Bodin's own interpretation would not have passed muster in, for example, Austin's eyes; and Austin's own definition is no longer adequate for expressing the intricacies of state-sovereignty in the interdependent world of the late twentieth century. The detailed examination of Baldus' theory will reveal that he was operating with a specifically late medieval concept of sovereignty which permitted the simultaneous acceptance of both universal and territorial forms, and that this concept can justifiably be termed one of sovereignty so long as the precise meaning attached to it is understood.