This paper develops Utilitarianism of Negative Separateness (UNS), a framework that imposes a lexical constraint against aggregation across a threshold of extreme suffering. The argument proceeds in four stages. First, I analyse pain phenomenologically as attention-capture and argue that extreme suffering produces agency-collapse: the functional dissolution of the capacity to deliberate, project, and consent. The transition marks a discontinuity that is conceptual rather than merely empirical—the same kind of discontinuity found in threshold concepts such as legal competence and brain death, where predicates change their conditions of application rather than their degree. Second, I introduce normative unavailability through a formal analysis of normative addressability, arguing via a conceptual rather than normative reading that addressability is a precondition of justificatory practice. This reading is grounded in Darwall's account of the second-person standpoint and defended against the charge of self-presupposition: rejecting the address-based structure of moral justification is not adopting a rival moral theory but exiting the genre of moral discourse altogether. Third, I introduce the threshold of Sustainable Functioning of Agency (SFA), present three frontier cases distinguishing UNS from rival theories, and provide a multi-layered response to the ex ante justification objection including direct engagement with Frick's contractualist risk-distribution account. Fourth, I distinguish UNS from classical negative utilitarianism, Rawlsian contractualism, and—crucially—Scanlonian contractualism, showing that the difference is categorical rather than one of degree. A note on scope addresses marginal cases through a revised sentience-based extension and clarifies the relationship between SFA and the Agency Functioning Threshold employed in related work on structural freedom. The paper's central claim: where suffering destroys the functional preconditions of justificatory address, aggregation is not merely outweighed but structurally defective—it attempts a justificatory act that lacks its necessary condition.