The concepts of sovereignty have been discussed throughout history, and are still actively debated. Its definition, concept, and application has changed throughout, especially during the Age of Enlightenment. The current notion of state sovereignty contains four aspects consisting of territory, population, authority and recognition. According to Stephen D. Krasner, the term could also be understood in four different ways:
Often, these four aspects all appear together, but this is not necessarily the case – they are not affected by one another, and there are historical examples of states that were non-sovereign in one aspect while at the same time being sovereign in another of these aspects. According to Immanuel Wallerstein, another fundamental feature of sovereignty is that it is a claim that must be recognised by others if it is to have any meaning: "Sovereignty is more than anything else a matter of legitimacy requires reciprocal recognition. Sovereignty is a hypothetical trade, in which two potentially conflicting sides, respecting de facto realities of power, exchange such recognitions as their least costly strategy."
Ulpian was expressing the idea that the Emperor exercised a rather absolute form of sovereignty, that originated in the people, although he did not use the term expressly.
Classical Ulpian's statements were known in medieval Europe, but sovereignty was an important concept in medieval times. Medieval monarchs were not sovereign, at least not strongly so, because they were constrained by, and shared power with, their feudal aristocracy. Furthermore, both were strongly constrained by custom.