John Ruggie draws on Durkheim’s social facts and Giddens’ structuration theory to examine traditional international relations issues from a constructivist perspective. Constructs in the international system are dualistic in that they restrict actors, but are also created and recreated by these actors. Neo-utilitarianism, including neo-realism and neo-liberalism, focuses on how international constructs restrictive power, but overlooks the construction process. Ruggie proposes his own structural evolution theory by carefully analyzing the changes between the feudal state systems of the Middle Ages to the modern territorial state system. He proposes that changes in contact consistency between actors change the international system’s differential principle of units, and these changes likewise affect the international system.