The idea that international law constitutes a system is an unsurprisingly popular construction in the legal academy. This article argues that international lawyers have found in the International Court of Justice (hereafter the ICJ) and its sources-based and rules-based modes of legal reasoning the support and the necessary components to build (and sell) their much-cherished idea of an international legal system. As this article argues, such process of system-design rests on a fundamental irony. This irony lies with the fact that the ICJ itself has never proved very interested in system-design and always fell short of portraying international law as a legal system. The indifference of the Court to the idea of international legal system contrasts with the—carefully tailored—argumentative benefits which regional courts have associated with the idea of an international legal system. It is the aim of this article to examine how international lawyers constantly turn to the ICJ and its rules-based and sources-based modes of legal argumentation to seek support for their portrayal of international law as a system despite the ICJ’s common indifference for systemic thinking about international law.