Borderline personality disorder (BPD) is a severe psychiatric condition characterized by instability in identity, relationships, and affect. Individuals, with BPD typically lack a coherent sense of self, are highly sensitive to interpersonal stressors, experience intense fluctuations in mood, and frequently engage in impulsive and self-destructive behaviors. Although both empirical research and development of effective psychotherapy have evidently progressed over the past years, many aspects regarding the structure of experience and the life-world typical for persons with BPD are not yet fully understood. Somewhat surprisingly, phenomenological psychopathology has only recently started to pay more attention to the disorder. A comprehensive elaboration of the phenomenology of BPD is therefore still lacking. This article aimed to contribute to such a phenomenological understanding by focusing on what we think is an essential aspect that has yet not been sufficiently addressed: the background of safety. To clarify what this means, we depart from Sandler’s [<i>Int J Psychoan</i>. 1960;41:352–6] psychoanalytic concept and elaborate on it phenomenologically. This leads us to argue that the development of a background of safety requires a particular embodied presence of others, which, in turn, contributes to the constitution of a safe we-space, a shared and familiar environment providing a matrix for the experience of a stable world. However, even when established, the background of safety remains in need of a continuous reconfirmation through corresponding experiences within a sufficiently reliable and controllable environment. The background of safety is vulnerable and open to (interpersonal) disruptions like trauma or neglect. In BPD, we suggest 3 aspects regarding the phenomenology of the background of the safety need to be considered: first, typically, patients with BPD did not develop a robust background of safety in infancy; second, weakening of the background of safety gives rise to symptoms and dynamics typical for BPD; third, these symptoms and dynamics further undermine the possible development of a background of safety in adult life and thus gravitate toward a petrification of the borderline condition, a “stable instability.” To conclude, we examine whether this concept should be understood as a <i>trouble générateur</i> and, last, consider its clinical implications.