The story of the development of the borderline concept has been well told by Gordon Claridge (1985) and our own view of borderline states has been enriched by his excellent Origins of Mental Illness. The borderline concept arose because many people with psychiatric disorders cannot be conveniently pigeon-holed into the neat categories devised for them by the diagnosticians. In practice, one may often encounter individuals presenting such a complex clinical picture that it is not easy to decide whether they are mildly psychotic, severely neurotic, or suﬀering from a long-standing personality disorder. Originally introduced by psychoanalysts to describe a group of patients who proved resistant to therapy, or indeed were made worse by it, the borderline concept has come to be applied to individuals whose abnormal personalities seem to combine features of neurotic and psychotic symptomatology. Before proceeding any further, therefore, we should brieﬂy examine these diagnostic categories.