The present paper first examines Stephen D. Krasner's analysis of state sovereignty and some criticisms raised against it. The examination shows that the issue of change versus continuity lies at the bottom of the opposition between Krasner and his critics. The opposition can be generalized into that between the mainstream international relations theorists and its critics, especially constructivism, because the former assumes that the international system is unchanged. The opposition boils down to the difference in the time span. The mainstream theory also assumes that state, the unit of the international system is unitary, thus excluding important actors and factors of domestic politics from international relations. This assumption is again severely criticized. Helen V. Milner proposes an extension of the mainstream theory to include domestic politics, on the basis of the criticism of the limit of the theory. This paper examines her proposal as an example of the criticism. It shows that the necessity of the inclusion or exclusion of domestic actors or domestic politics depends upon the particular research objective which a researcher pursues, and that the debates about the inclusion of domestic politics are reduced to the difference in the level of abstraction or aggregation which a theory aims at. Thus the differences between the mainstream theorists and their critics amount to those in the time frame and the scope of variables. In other words, the difference in the level of abstraction or aggregation lies at the root of their differences. To be sure, this paper ends up with a cliche, which tells us only that the appropriate level of abstraction or aggregation depends crucially on a particular theme or research agenda. But, it does tell us that, in an international relations theory, we must yet to decide which level of abstraction is suitable for what research purpose. A clear understanding of this will be much more fruitful than the mere exchange of criticisms and counter-criticisms.