Nosology, taxonomy and the classification conundrum of the functional psychoses.

The theory underlying medical classification is not always given sufficient regard. Here systematic methods developed in biology are summarised and compared with parallel developments in nosology. The classification of the functional psychoses is discussed as an example of a methodological problem in medicine. We consider that cladism could have a useful application in nosology, and could lead to classifications based on the concept of interrelationships of disease.