ABSTRACT
Whenever natural variations are seen in some phenomenon there is always a need to classify them – to arrive at what in biology is termed a taxonomy, an orderly descriptive system for bringing together subsets of examples that share similar features. In the case of psychological disorders, attempts to construct such a taxonomy – in this case called a nosology – has traditionally been in the hands of psychiatrists. Consequently, as will be evident from our introductory chapter, the form and thinking behind psychiatric nosologies have been very much dictated by a medical model, a belief in the need to identify distinct psychological diseases. This has often attracted criticism. But it should not blind us to the fact that, irrespective of how it is done, some way of systematically distinguishing between the various psychological disorders is required. Why is this? There are three main reasons.
