On several occasions in the history of psychiatry there have been resurgences of support for the idea that one or two broad categories should suffice for the classification of mental disorders. Differences between mentally disordered patients could then be attributed, not to different psychological disorders, but to differences in their personalities and circumstances. Like many ideas in psychiatry, this one had its origins in the German-speaking world. It is sometimes known by the German name of the Einheitspsychosen hypothesis: the hypothesis that mental disorders form a unified category – an Einheit – with any further divisions of that category being arbitrary. It would be natural to categorize all the crows together in one group, with other birds in others. The scientific status of an enquiry therefore seems to depend upon the non-arbitrary status of its categories.