  Realism encompasses a variety of approaches and claims a long theoretical tradition. Among its founding fathers, Thucydides, Machiavelli and Hobbes are the names most usually mentioned. Twentieth-century classical realism has today been largely replaced by neorealism, which is an attempt to construct a more scientific approach to the study of international relations. Both classical realism and neorealism have been subjected to criticism from IR theorists representing liberal, critical, and post-modern perspectives.