Theory synthesis is not only possible and desirable but is constitutive of any coherent understanding of international relations as a progressive and empirical social science. Numerous interesting proposals exist for formulating and empirically testing multitheoretical propositions about concrete problems in world politics. Below the reader will find a set of basic principles that should underlie testable theory syntheses. Yet other contributors to this forum—Friedrich Kratochwil, Yosef Lapid, Iver Neumann, and Steve Smith—do not share this openness to theory synthesis; their views range from deep skepticism to outright rejection. The real issue between us is whether pluralism among existing theories ought to be preserved for its own sake, as these colleagues believe, or whether theories ought to be treated as instruments to be subjected to empirical testing and theory synthesis, as this author maintains.