Legitimacy is an important concept in political theory, and the theory of legitimacy is a significant component of political theory.Although it was during the modern times that the theory of legitimacy was explicitly put forward and studied systemically, people had begun to think about it ever since there was state or political society in human history.The concept of legitimacy is indispensably connected with a state's political life and the theory of legitimacy discusses the political phenomenon of the rise and fall of a state's regime.Although there were differences in the theory of legitimacy between ancient political thinkers and those after the Renaissance, they generally followed the tradition of rational constructivism.After the 19th century, political thinkers shifted from rational constructivism to empiricism in their inquiry into the theory of legitimacy.In contemporary studies, however, rational constructivism and empiricism are combined together.