The term psychosis refers to mental disorders which affect widespread areas of mental functioning and which are often, but not necessarily always, accompanied by the hallmark symptoms of delusions, hallucinations, and incoherence of speech. The qualification of functional serves to distinguish two such states, schizophrenia and manic–depressive psychosis, from the so-called organic psychoses of delirium and dementia – which also affect mental functioning in a global way and which can themselves be accompanied by delusions, hallucinations as well as many other psychiatric symptoms. The distinction turns on the presence or absence of a single clinical feature, cognitive impairment. This is always present, if sometimes subtle, in delirium and dementia, but the absence of memory impairment, disorientation and other evidence of disturbed intellectual function is invariably held to be a prerequisite for the diagnosis of schizophrenia or manic–depressive psychosis.

Why, then, is there a chapter on functional psychosis in a book about memory disorders? The answer to this question lies in the history of psychosis and the uneasy nosological position occupied by functional psychosis, in particular, over the course of the twentieth century. Established in the nineteenth century (see Berrios, 1987, 1996; Beer, 1996), the concept of psychosis underwent various modifications and subdivisions in accordance with the currents of thought of the period. One important distinction that appeared was between exogenous and endogenous psychoses: the former were due to toxic and infectious agents whereas in the latter the cause lay principally in the individual, often in an apparent hereditary predisposition.