  The borderline patient is pictured as having a specific stable pathological personality organization, characterized by a specific kind of underlying structural configuration. It should be noted that the stability here refers to the structural configuration, and not to mood and behavioral fluctuations. The structural configuration is viewed as including a distinct kind of ego and superego functioning, and a distinct pattern of instinctual drive organization.
This configuration is quite stable and resistant to change except through intensive psychotherapeutic intervention or with the passage of much time. The borderline patient is viewed as having one of four structural configurations, within which all patients can be classified diagnostically. Thus, the borderline structural configuration, along with the normal-neurotic, the narcissistic, and the psychotic, provide first-level "structural" diagnoses under which all other descriptive diagnoses fall.
Although the structural configuration includes a specific kind of ego and superego functioning, as well as a specific pattern of instinctual drive organization, the focus for diagnostic purposes will be largely on the ego. Relying on the model of Beres (1956) and Bellak (1958), the borderline patient's ego will be viewed in accordance with the various ego functions. The borderline individual is thus described as having a particular ego structure, consisting of a specific pattern of relative ego strengths and underlying ego weaknesses. The relative ego strengths are as follows: * The relative intactness of reality testing; * The relative intactness of thought processes; * The relative intactness of interpersonal relations; * The relative intactness of the adaptation to reality. It must be stressed that these four strengths are only relative; they easily break down to various degrees in various situations.
Because these four relative strengths stand out superficially, they enable the borderline individual to present a fairly "normal" appearance. These relative strengths, particularly the first two, most clearly differentiate the borderline from the more psychotic individual. The underlying ego weaknesses are as follows: * The combination of poor impulse control and poor frustration tolerance; * The proclivity to use primitive ego defenses; * The syndrome of identity diffusion; * Affective instability. In contrast to the strengths, which stand out on a superficial level, these weaknesses only become clearly apparent with in-depth understanding. Except during regressed states, a detailed history or a relationship over time is needed for these weaknesses to clearly emerge.
Because these weaknesses are beneath the surface and are not detected superficially, they do not detract from the borderline's appearance of normality. These underlying weaknesses, however, most clearly differentiate the borderline from the more neurotic individual. 
