In everyday speech the word ‘illness’ is used loosely. Similarly, in psychiatric practice the term ‘mental illness’ is used with little precision, and often synonymously with ‘mental disorder’. In this context, the terms ‘mental’ and ‘psychiatric’ are also used interchangeably.

A good definition of mental illness is difficult to achieve, for both practical and philosophical reasons, as outlined here. In routine clinical work the difficulty is important mainly in relation to ethical and legal issues, such as compulsory admission to hospital. In forensic psychiatry the definition of mental illness (by the law) is particularly important in the assessment of issues such as criminal responsibility.

Diverse discussion of the concepts of mental illness can be found in Lazare (1973), Kendell (1975), Zachar and Kendler (2007), and Tyrer (2013).

Definitions of mental illness

Many attempts have been made to define mental illness, none of which is satisfactory or uniformly accepted. A common approach is to examine the concept of illness in general medicine and to identify any similarities or analogies with mental illness. In general medicine there are five types of definition:

● Absence of health. This approach changes the emphasis

of the problem but does not solve it, because health

is even more difficult to define. The World Health

Organization, for example, defined health as ‘a state

of complete physical, mental and social well-being,

and not merely the absence of disease or infirmity.’ As

Lewis (1953) rightly commented, ‘a definition could

hardly be more comprehensive than that, or more

meaningless.’ Many other definitions of health have

been proposed, all equally unsatisfactory.

● Disease is what doctors treat. This definition has the

attraction of simplicity, but does not really address the

issue. The notion that disease is what doctors can treat

has somewhat more merit, since there is evidence

that, as a medical treatment for a condition becomes

available, it becomes more likely that the condition will be regarded as a disease (Campbell et al., 1979).

● Biological disadvantage. The idea of defining disease in terms of biological disadvantage was proposed by Scadding (1967), and is the most extreme biomedical view of disease. Scadding never defined biological disadvantage, but the term has been used in psychiatry to include decreased fertility (reproductive fitness) and increased mortality. Viewing disease in terms of ‘evolutionary disadvantage’ is a similar concept (Wakefield, 1992).

● Pathological process. Some extreme theorists, most notably Szasz (1960), take the view that illness can be defined only in terms of physical pathology. Since most mental disorders do not have demonstrable physical pathology, according to this view they are not illnesses. Szasz takes the further step of asserting that most mental disorders are therefore not the province of doctors. This kind of argument can be sustained only by taking an extremely narrow view of pathology. It is also arbitrary, based on current knowledge, and is increasingly incompatible with the evidence of a genetic and neurobiological basis to the major psychiatric disorders, and their associated morbidity and mortality.

● Presence of suffering. This approach has some practical value because it defines a group of people who are likely to consult doctors. A disadvantage is that the term cannot be applied to everyone who would usually be regarded as ill in everyday terms. For example, patients with mania may feel unusually well and may not experience suffering, although most people would regard them as mentally ill.

Biomedical versus social concepts

The above concepts may be divided into those that view mental illnesses in purely biomedical terms, and those that consider them to be social constructs or value judgements. This debate is still ongoing, and depends in part on one’s opinion about their

aetiology, but it is now generally accepted that value judgements play a part in all diagnoses, even if the disorders themselves are considered from a biomedical perspective (Fulford, 1989). For example, beliefs and emotions are central to most psychiatric disorders, yet it is a value judgement as to whether a given belief or emotion is ‘reasonable’ or ‘unhelpful’ for a given individual in their particular social context, and therefore what, if any, diagnostic significance it has. Would we use ‘useful’ or ‘dysfunctional’ to decide whether a belief was ‘illness’? Would ‘normal’ or ‘abnormal’ be better?

Impairment, disability, and handicap

It is useful in medicine, and particularly in psychiatry, to describe and classify the consequences of a disorder. This approach is related to the concept of disease as involving dysfunction (Wakefield, 1992), as incorporated into the definitions of mental disorder used in ICD-10 and DSM- 5 (see below). Three related terms, derived from medical sociology and social psychology, are used to describe the harmful consequences of a disorder.

● Impairment refers to a pathological defect—for

example, hemiparesis after a stroke.

● Disability is the limitation of physical or psychological function that arises from an impairment—for

example, difficulties with self-care that are caused by

the hemiparesis.

● Handicap refers to the resulting social dysfunction—

for example, being unable to work because of the

hemiparesis.

Incapacity may be seen as another harmful consequence of illness, although the term usually refers in a legal sense to the effect that illness has on one’s competence to make treatment decisions, as enshrined in the United Kingdom by the Mental Capacity Act (see Chapter 4).

Diagnoses, diseases, and disorders

The term ‘diagnosis’ has two somewhat different meanings. It has the general meaning of ‘telling one thing apart from another’, but in medicine it has also acquired a more specific meaning of ‘knowing the underlying cause’ of the symptoms and signs about which the patient is complaining. Underlying causes are expressed in quite different terms from the symptoms. For example, the symptoms of acute appendicitis

are distinct from the idea that will form in the mind of the doctor that the appendix is inflamed and producing peritoneal irritation. To be able to make a diagnosis of this type is, of course, satisfying for the doctor and useful for the patient, since it immediately suggests what investigations and treatment are needed. Its clear utility also makes redundant most theoretical or philosophical concerns about classification. Unfortunately, for most psychiatric patients it is rarely possible to arrive at this type of diagnosis, the only exception to this being, by definition, ‘organic’ psychiatric disorders (see page 26).

The lack of clear disease categories, in a medical sense, has led to the use of the more general term ‘disorder’. The definition of a psychiatric disorder in ICD-10 is:

…a clinically recognizable set of symptoms or behavior associated in most cases with distress and with interference with personal functions. Social deviance or conflict alone, without personal dysfunction, should not be included in mental disorder as defined here.

(World Health Organization, 1992b, p. 5).

The DSM-5 definition of a mental disorder is longer but similar:

…a syndrome characterized by clinically significant disturbance in an individual’s cognition, emotion regulation, or behavior that reflects a dysfunction in the psychological, biological, or developmental processes underlying mental functioning. Mental disorders are usually associated with significant distress or disability in social, occupational, or other important activities. An expectable or culturally acceptable response to a common stressor or loss, such as the death of a loved one, is not a mental disorder. Socially deviant behavior (e.g. political, religious, or sexual) and conflicts that are primarily between the individual and society are not mental disorders unless the deviance or conflict results from a dysfunction in the individual, as described above.

(American Psychiatric Association, 2013a, p. 20).

Despite the similarity, there is an important difference between the two definitions. ‘Interference with personal functions’ in ICD-10 refers only to such things as personal care and one’s immediate environment, and does not extend to interference with work and other social roles. In DSM-5, as in the extract above, impairment refers to all types of functioning.

Both definitions illustrate that most psychiatric disorders are based not upon theoretical concepts, or presumptions about aetiology, but upon recognizable clusters of symptoms and behaviors. This reliance explains much of the debate about the reliability and validity of the categories being classified, as will be discussed later in this chapter.