International law is ‘fragmented’ into a number of specialised systems such as trade, environment and human rights (see Koskenniemi 2006; Pauwelyn 2003). Each system has developed its own analytical framework (concepts and relationships), normative concerns (values and interests) and institutions to understand itself and the world around it. This makes the systems appear as autonomous from both the broader system of general international law and each other (Koskenniemi 2006 at paras 53-4). However, these separate systems are often applied to the same sub­ stantive issues and situations. With no single system able to claim analytical or normative superiority, there appears no method for commensuration. In the context of World Trade Organization (WTO) law it is possible to identify three common responses to the systemic fragmentation of inter­ national law. The first is ‘openness’: arguing that external international law can be used in particular as a defense against the violation of WTO law, but also generally to emphasise the holistic nature of the international legal system (Pauwelyn 2003, 2001: 536). The second common response is ‘closure’: arguing that the WTO can only use WTO law, beyond which WTO law should not reach (see Trachtman 1999: 855-61). Openness and closure therefore appear to be mutually exclusive pro­ positions on how the WTO should respond to fragmentation in its applica­ tion of external international law. Proponents of openness criticise closure

for not seeing the vast jurisdiction and norms available to the WTO adju­ dicating bodies in using external international law to come to a decision in WTO law. Proponents of closure counter that proponents of openness see too vast a jurisdiction and the corresponding norms available to the WTO adjudicating bodies in their decision­ making process. Each proposi­ tion appears to contain its equal and justifiable counter­ argument. In fact the propositions of openness and closure are what can be termed ‘nested oppositions’ (Balkin 1990: 1671-7, 1683-6, 1994: 396-8), that is while appearing contradictory, they start from a shared premise. I argue that neither the openness nor the closure of WTO law can be maintained simultaneously because there is constant oscillation between the two propositions. The third common response is to attempt to reconcile openness and closure by claiming that WTO law overrides external international law because of a conflicts clause inferred in WTO law (for example, Bartels 2001). But this middle ground is untenable, because at any given moment the view of WTO law takes on the characteristics of either ‘closure’ or ‘openness’. Rather than adhere to openness or closure, I argue that it is more productive to build upon the concept of ‘operational closure’ origin­ ated by Gunther Teubner (1987a and 1993), and Julia Black (1996). The approach informs the conceptual framework of constrained openness that I put forward to understand WTO law and its interaction with external international law. I explain and supplement the framework of constrained openness by reference to Italo Calvino’s (1997) Le cittá invisibili (Invisible Cities). The analysis provides the basis for a re­ examination of the existing propositions on the interaction between WTO law and external law. In conclusion, I posit that constrained openness would provide a more nuanced understanding of the interaction between WTO law and external international law.