Three distinct trends in modern psychiatry distract from a proper concern with the phenomena of mental disorders. Pressures of time and heavy case loads may stand in the way of detailed exploration of mental states. A quick scan for a few critical markers — mood disturbance, thought disorder, delusions, hallucinations, cognitive dysfunction — often seems to suffice for currently accepted diagnostic classifications and treatment decisions. And thirdly, clinical science in psychiatry is dominated by neurological, epidemiological, and psychosocial themes which commonly disregard the intricate realities of abnormal psychic phenomena.

It was a common view among 19th century historians and clinicians that the study of delusions was the study of insanity itself (Ball & Ritti, 1881). At the beginning of the 20th century, Jaspers rendered this insight into a cliche (Jaspers, 1963). The nature of the link between delusion and insanity, however, has continued to confuse scholars, particularly those writing in the English language (Ireland, 1885; Arthur, 1964; Moor & Tucker, 1979; Winters & Neale, 1983). German (Huber & Gross, 1977), French (Ey, 1950) and Spanish (Cabaleiro Goas, 1966) writers have fared better; unfortunately, much of their work remains inaccessible to English-speaking psychiatrists. This is one of the reasons why, in Anglo-Saxon psychiatry, it has been suggested that the ‘definitive‘ view on delusions started with Jaspers and the Heidelberg school (Hoenig, 1968). This suggestion is misleading (Berrios, 1991), for by 1912, when Chaslin published his great work on descriptive psychopathology, all the distinctions nowadays attributed to Jaspers had already been made. Indeed, the rare efforts made to escape from the ‘pathological belief view were ignored (Southard, 1916).

Delusions are among the most common psychiatric symptoms, but the psychological processes involved in their formation and maintenance remain a matter of controversy. This paper briefly reviews competing theories, particularly those involving abnormalities of perception and reason. An investigation of reasoning in deluded schizophrenic and paranoid subjects is presented, followed by a tentative model of belief formation in which abnormalities or biases in reasoning and perception, implicated in the formation of some delusions, are highlighted.

“Extreme psychotic states offer a human parable … patients see into the depths which do not belong so much to their illness as themselves as individuals with their own historical truth … in psychotic reality we find an abundance of content representing fundamental problems in philosophy …. The philosopher in us cannot but be fascinated by this extraordinary reality and to feel its challenge.” (Jaspers, 1963)

According to Kendell (Kendell, 1975) the classification of psychiatric disorders should “cleave nature at its joints”. The central task of nosology then is to discover ‘nature's joints‘. There are, generally speaking, two major approaches to this task, which as Kendell has outlined, trace their origins to the epistemological debates between Platonic and Aristotelian schools. The first is the empirical approach from Locke and Hume, which is currently in ascendancy as exemplified by DSM—III and DSM—III—R, (American Psychiatric Association, 1980, 1987) while the second can be regarded as the ‘rational‘ approach deriving from the philosophy of Kant (Varghese, 1988). Table 1 outlines the differences between nosological approaches.

In schizophrenia, where they find their fullest expression, delusions are an extraordinary phenomenon. They can take a bewildering variety of forms, persecutory, grandiose, hypochondriacal, referential, and so on. They can be fixed and immutable, or alternatively may shift and change to the point of what Bleuler termed a delusional chaos. Finally, their degree of implausibility starts from the inherently unlikely, ranges through the patently absurd and bizarre, and not infrequently comes to violate common sense at its most elementary in fantastic delusions.

I write as one who has grappled with improving the classification of the psychoses, but I believe that the strategy of developing a set of descriptive concepts, and using such preliminary groupings to search for the primary causes of the ‘diseases‘ defined (genes, brain damage, social and psychological stressors, etc.) is insufficiently ambitious. Psychiatry has been too reductionist in its attitude to both the causes and the treatment of symptoms (such as delusions), placing the entire emphasis on investigating and treating the ‘underlying disease‘, not the delusional process itself. Internal medicine has often been able to unfold the whole disease process from primary causes and preconditions through disordered physiology to symptoms. In psychiatry it is also important to understand the pathogenesis of symptoms, that is, to specify the stages in the transposition of stress and cerebral pathology into the phenomena of mental illness.

The psychiatric section, entitled ‘Mental, Behavioural and Developmental Disorders‘ of the International Classification of Diseases, is currently in the process of revision, and ‘ICD—10‘ will shortly become available. This revision will be based partly on its immediate predecessor, the 9th Revision of the International Classification of Diseases (ICD—9; World Health Organization, 1978), and also upon the American Diagnostic and Statistical Manual (DSM—III—R; American Psychiatric Association, 1987). ICD—10 describes and lists symptoms required for making each specific diagnosis and it also refers to inclusions and exclusions. The symptoms themselves, however, are not defined nor described, and an ill-informed method of evaluating symptoms or a lack of thoroughness in their ascertainment will result in mistaken diagnoses. The descriptive psychopathologist clearly has a part to play in encouraging accurate usage.

Who are mad, those in lunatic asylums or the rest of us, pruning our inane rituals outside? What is reality, what delusion? These are the basic questions of the human predicament posed by Pirandello's play Henry IV, now revived on the London stage with Richard Harris's mesmeric central performance as the alienated Everyman, who finds fulfilment in posing as the tormented medieval king.

“And the end of our exploring Will be to arrive where we started And know the place for the first time.” (Eliot, 1952, p. 145)

In preparing this report I made the mistake of looking back in the literature, the literature not only of delusions, but also including some more general works of the earlier clinical writers, such as Birnbaum (1923), Conrad (1958), Jaspers (1963) and Kretschmer (1927). It was a humbling experience; these were obviously people who knew something about patients and about the realities of symptoms. For these writers, notions such as delusions were not just abstractions, not even just abstractions with operational definitions. These authors described real experiences of people, and their descriptions reflect the importance of studying patients from close clinical contact.

The monodelusional disorders correspond closely to the Kraepelinian description of paranoia (Kraepelin, 1921), and to the DSM—III—R category of “delusional (paranoid) disorder” (American Psychiatric Association, 1987). The essential feature is a stable delusional system, and the encapsulated quality of the delusion is characteristic. The rest of the personality is notably well preserved, but the delusional system, despite its encapsulation, takes over much of the individual's way of life. There are profound and striking secondary changes in affect, attitude and logic when the patient switches from the normal to the delusional mode or vice versa.

The basic concept characterising the delusional misidentification syndromes (DMS), particularly the Capgras variety, is the concept of double, or its equivalents. It seems that this concept is prominent in all cultures and this explains the amazing universality of these delusions. Cases of DMS have been reported in practically all parts of the world. In fact, most patients actually mention the word double (or σωσιας, sosie etc.) or refer to transformations (metamorphose, μεταμoρΦωσεις and the like). The preoccupation, and even enchantment, of man with the notion of doubles from time immemorial in all cultures, as reflected in the myths, traditions, literature, beliefs and metaphysical motions, has been dealt with extensively elsewhere (Christodoulou, 1986a).

For over 50 years, Capgras' syndrome (Capgras & Reboul-Lachaux, 1923) was considered a psychiatric curiosity, fit only for inclusion among other ‘rarities’ in psychiatric textbooks and explicable only in psychodynamic terms. In the last few years, however, the number of publications on the topic has increased enormously, and three solid facts have emerged which give a clue to its nature. In this article, I shall discuss these three facts and, in the light of them, propose a psychological and neuropsychological explanation for the syndrome.

In 1871, Charles Darwin published The Descent of Man and Selection in Relation to Sex. The first part of this volume made explicit the conclusion that had been implicit in the Origin of Species published 12 years earlier, that man indeed was descended from the great apes.

The patient who develops the delusional conviction that he/she is infested with small organisms, such as mites or insects, has been recognised in the medical literature for over a century. A wide range of descriptive terms — ‘dermatophobia’ ‘acarophobia’, ‘parasitophobia’ and ‘entomophobia’ (Table 1) have been applied to this symptom. Such terminology is now inappropriate but it has continued to be used until relatively recently: Pope (1970), for example, defined parasitophobia as “a delusional state in which there are sensations of infestation of the skin by insects”.

The condition termed ‘delusional atmosphere‘ belongs to the ‘precursor phenomena‘ of delusion and has always attracted special attention because its peculiarities evoke the hope of obtaining better insight into the conditions governing the emergence and formation of delusion. The following considerations represent a review of the most important topical hypotheses about the nature of the delusional atmosphere and its progression into finalised delusional elaborations as seen in the perspective of the psychopathology of German-speaking countries.

“Since time immemorial, delusion has been taken as the basic characteristic of madness. To be mad was to be deluded” (Jaspers, 1963, p. 93).

This paper reviews the phenomenology of delusional perception and discusses its differentiation from similar phenomena, its diagnostic value, and its pathogenesis. The study of delusional perception is of particular interest, because, unlike other delusions, its identification rests on form as well as content. Also, it is one of the first-rank symptoms of schizophrenia, and its mode of onset, which is an important part of the phenomenon, may help us to understand how other delusions develop.

The significance of delusions for psychiatry is well recognised. Scientifically respectable, delusions are reliably identifiable clinically (Wing et al, 1974), widely acknowledged across diverse cultural norms as genuine symptoms of illness (Wing, 1978), and central to the historical notion of insanity (Jaspers, 1913). What is not so well recognised, however, is that delusions are also highly significant for philosophy. This is not well recognised either by philosophers or by doctors. Yet the key to recognising it lies not in some obscure philosophical analysis of delusions but in a feature of their everyday clinical phenomenology.