A new developmental model of borderline personality disorder (BPD) and its treatment is advanced based on evolutionary considerations concerning the role of attachment, mentalizing, and epistemic trust in the development of psychopathology. We propose that vulnerability to psychopathology in general is related to impairments in epistemic trust, leading to disruptions in the process of salutogenesis, the positive effects associated with the capacity to benefit from the social environment. BPD is perhaps the disorder par excellence that illustrates this view. We argue that this conceptualization makes sense of the presence of both marked rigidity and instability in BPD, and has far-reaching implications for intervention.