Any discussion of the classification of psychiatric disorders should begin with the frank admission that any definitive classification of disease must be based on aetiology. Until we know the causes of the various mental illnesses, we must adopt a pragmatic approach to classification that will best enable us to care for our patients, to communicate with other health professionals and to carry out high-quality research.

In physical medicine, syndromes existed long before the aetiology of these illnesses were known. Some of these syndromes have subsequently been shown to be true disease entities because they have one essential cause. Thus, smallpox and measles were carefully described and differentiated by the Arabian physician Rhazes in the tenth century. With each new step in the progress of medicine, such as auscultation, microscopy, immunology, electrophysiology and so on, some syndromes have been found to be true disease entities, while others have been split into discrete entities, and others still jettisoned.