A disputed concept (see, for example, the entries elsewhere in this dictionary on Laing and anti-psychiatry) founded on the everyday contrast between mind and body which, when applied to illness, generates an opposition between two contrasting types of illness—mental and physical. Mental illnesses are illnesses characterized by the presence of mental pathology: that is, disturbances of mental functioning, analogous to disturbances of bodily functioning. Like physical illness the concept is, therefore, fundamentally evaluative and linked to issues of social control and regulation. The disturbances of thought and feeling that characterize mental illness, such as delusions, hallucinations, excessive elation, or depression, are often associated with behaviour that is considered bizarre, awkward, disruptive, or disturbing. It is this disturbed and disruptive behaviour that, more than anything else, leads to mental illness being treated as a very distinctive form of illness, requiring special services and attention. What seems to be particularly problematic within society is the apparent irrationality and loss of reason that mental pathology involves. Mind and reason are the distinctive hallmarks of human beings, and their loss (full or partial), if not taken to be a sign of supernatural powers, is generally viewed as profoundly disturbing and threatening. Mental illness is therefore usually more stigmatizing than other forms of illness (obvious exceptions are diseases such as AIDS, other sexually transmitted diseases, and to a lesser extent cancer).
Historically, the medical concept of mental illness has its basis in lay judgements of mental states, which were embodied in notions such as insanity and lunacy—real madness—as well as in concepts such as ‘troubled in mind’, ‘mopish’, and ‘distracted’, which encompassed the less severe forms of psychological disturbance. Then as now these terms were applied to persons whose behaviour seemed in some way inexplicable or irrational. Deviant or delinquent it might be, but it could not be understood as readily as the usual forms of delinquency, often because it involved a rejection of what was highly valued in society. Present-day medical conceptions of mental illness are still intimately linked to lay judgements of what is rational, reasonable, and appropriate. However, psychiatry has embraced and transformed the everyday lexicons, classifying and listing a diverse set of mental illnesses. These range from conditions such as Alzheimer's disease, which are known to involve brain pathology, through the archetypical mental illnesses such as schizophrenia and manic depression (which belong to the group of psychoses), to conditions such as anxiety states, phobias, and obsessions (frequently termed neuroses), as well as to the so-called behaviour disorders such as alcoholism, anorexia nervosa, drug addiction, and sexual deviations.
Psychiatrists’ lists of mental illnesses provide a formal specification of the boundaries of mental illness, however the boundaries are changing and contested. The distinction between mental illness and physical illness is itself highly problematic. It is most obviously made in terms of manifest pathology, but is often not clear-cut, with many illnesses having mental and physical symptoms; once we turn to causes the distinction is even more problematic and the idea of two mutually exclusive categories of illness soon founders. An identifiable mental pathology may well have physical causes, as in the case of Alzheimer's disease; equally, some physical pathologies such as ulcers have mental causes (as the concept of psychosomatic illnesses allows). Indeed, the interrelation of mental and physical has frequently been used to justify attempts to integrate mental health services with other health services. In practice, where the boundary is set between mental and physical illness is a matter of convention, and depends on ideas about causation as well as on the extent of the manifest mental and behavioural problems.