Mentalization is the process by which we implicitly and explicitly interpret the actions of ourselves and others as meaningful on the basis of intentional mental states (e.g., desires, needs, feelings, beliefs, and reasons). The capacity develops during childhood within the context of an attachment relationship. It is suggested that the borderline patient shows a reduced capacity to mentalize and that this has resulted from disruption of the attachment relationship because of adverse interaction between biological and environmental factors.

We mentalize interactively and emotionally when with others. Each person has the other person's mind in mind (as well as their own) leading to self-awareness and other awareness. We have to be able to continue to do this in the midst of emotional states but borderline personality disorder is characterised by a loss of capacity to mentalize when emotionally charged attachment relationships are stimulated. This leads to misinterpretations about the motives of others, difficulty in managing emotional states, and self-destructive behaviour as the individual seeks some stability and tries to re-gain some mentalizing capacity. Therapy has to help a patient develop and maintain mentalizing even when emotional states are aroused. Some therapeutic techniques will be described to aid this process and some principles discussed which guide the naïve therapist on when to give which intervention.