This chapter critiques the dualist methodological assumptions underpinning the study of identity in IR, showing how both Cartesian (positivist) and post-Cartesian (postpositivist) approaches remain grounded in a shared commitment to separability, despite their differences between fixity and fluidity. While positivist approaches stabilise identity as a bounded object and postpositivist approaches render it contingent and relational, both presuppose analytically distinct units as the starting point of enquiry, thereby limiting the capacity to apprehend identity as a processual phenomenon. In response, the chapter develops Beyond-Cartesianism as a nondualist methodological framework that refuses separability as the starting condition of analysis and reconceptualises identity as an emergent articulation within a non-separable field, grounded in a principle of unity-in-difference in which differentiation and interconnection are co-present. This framework is operationalised through non-separable processual identity analysis (NPIA), which enables the study of identity as it unfolds through multi-sited and multidirectional processes across shifting local-global contexts. Applied to the study of migration, the chapter demonstrates how NPIA reorients analysis from fixed categories and bounded domains towards the ongoing constitution of identity-in-motion, thereby enabling a post-binary understanding of identity in international politics.