The Constant Function at the Other: A Structural Model of Belief Maintenance in Borderline Personality Disorder

This paper proposes that the interpersonal patterns characteristic of Borderline Personality Disorder (BPD)—idealization, devaluation, identity disturbance, and abandonment sensitivity—can be understood as expressions of a specific belief-maintenance architecture. Drawing on a structural model of persistent belief-like narrative attributions maintained for regulatory purposes (Bahtışen, 2026a), the paper identifies a three-component system: (1) an invariant attribution structure exculpating the self regardless of surface presentation, (2) the assignment of an impossible regulatory function to another person defined in what it must deliver but not in how, and (3) rapid oscillation between two incompatible narrative frames, each internally consistent but too brief to be tested against reality. The paper introduces the distinction between first-order and second-order constant functions to account for BPD's characteristic instability. Five predictions are derived and evaluated against peer-reviewed literature: splitting as rapid narrative cycling, the partner as externally installed agent, a cortisol-prefrontal impairment pathway, self-blame as cycle fuel rather than exit, and crisis as belief-intensifier. Clinical literature is broadly consistent with all five predictions, though the model's contribution varies from genuine structural novelty to reframing of established insights. The paper addresses equifinality across therapies with different mechanisms, proposes novel testable predictions—including oscillation as measurable second-order invariance and self-blame intensity as a function of cycle count—and specifies falsification conditions, offering a structural complement to existing etiological accounts of BPD.