This paper establishes a transcendental result and proposes a formal articulation of it. The transcendental result: any intelligible account of determinate thinghood presupposes contrastive individuation. This claim has exclusionary force: it rules out Leibnizian complete-concept individuation and haecceitism, both of which hold that individuation requires no reference to contrast. The paper argues, through step-by-step analysis, that both accounts re-import contrastive structure when their individuating resources are pressed for determinacy — including the specialist replies that simples are given with primitively intelligible content and that haecceities are primitively numerically distinct. The proposed formal articulation is boundary-language: inside, outside, and boundary as primary, explicitly separated from the transcendental result it articulates. The paper defines adequacy (three necessary conditions) and minimality (four criteria) separately before testing boundary-language against three rival formalizations in a fixed comparative table. Each rival is shown to presuppose already-determinate individuals or to deploy implicit contrastive structure. Boundary-language is offered as the minimal adequate formalization. Whether being itself is boundary-structured is the question Paper 5 addresses. This paper's contribution stands independently. This is the fourth paper in a series; the first three are available at DOI: 10.5281/zenodo.19430954, DOI: 10.5281/zenodo.19454844, and DOI: 10.5281/zenodo.19486190.
Nikita Shchevyev (2026) studied this question.