" Though diagnosis is unquestionably critical in treatment considerations for many severe conditions with a biological substrate (for example, schizophrenia, bipolar disorders, major affective disorders, temporal lobe epilepsy, drug toxicity, organic or brain disease from toxins, degenerative causes, or infectious agents), diagnosis is often counterproductive in the everyday psychotherapy of less severely impaired patients. Why? For one thing, psychotherapy consists of a gradual unfolding process wherein the therapist attempts to know the patient as fully as possible. A diagnosis limits vision; it diminishes ability to relate to the other as a person. Once we make a diagnosis, we tend to selectively inattend to aspects of the patient that do not fit into that particular diagnosis, and correspondingly overattend to subtle features that appear to confirm an initial diagnosis. What’s more, a diagnosis may act as a self-fulfilling prophecy. Relating to a patient as a “borderline” or a “hysteric” may serve to stimulate and perpetuate those very traits. Indeed, there is a long history of iatrogenic influence on the shape of clinical entities, including the current controversy about multiple-personality disorder and repressed memories of sexual abuse. And keep in mind, too, the low reliability of the DSM personality disorder category (the very patients often engaging in longer-term psychotherapy). "
― Irvin D. Yalom , The Gift of Therapy: An Open Letter to a New Generation of Therapists and Their Patients