How can we make up our minds on whether or not international organizations are different from the sum of their parts? Taking a step back from doctrinal analysis, this chapter explores how the challenges that international lawyers have faced in that regard correspond to broader themes in philosophical discourse on ontological reductionism. This chapter suggests that questions of existence are inherently relative in the sense that they only make sense when considered in relation to other entities that are already admitted as non-redundant. Thus, the key to assessing the distinctiveness of international organizations is to first uncover the rationale that international law employs in buttressing their members as ‘real’ entities and then examine whether it can be equally applied to international organizations.