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• not only mental illness, which is generally temporary
  
• but also learning disability and personality disorder, which are lifelong.

The term ìmental disorderî is thus a general one; ìmental illnessî is just a type of mental disorder. In other words, mental disorder applies to many patients, but only some of them have a mental illness.

Psychiatry in fact has no agreed short definition of mental disorder, in the sense of a brief form of words able to encompass the whole field: it is just too big and too diverse.

MENTAL DISORDER INCLUDES, FOR EXAMPLE:

• those born with learning disability
  
• childhood conditions such as autism
  
• adults with permanently abnormal characters (personality disorders)
• illnesses such as depression and schizophrenia
• illnesses of later life where there is physical degeneration of the brain, such as Alzheimerís
• some aspects of habits such as alcohol and drug misuse
  
• some aspects of behavioural problems such as criminality.
  

The ICD 10, which stands for the 10th revision of the World Health Organisationís International Classification of Diseases, has Mental and Behavioural Disorders as its Chapter V.

But it does not offer a definition of the topic, proceeding without any introduction to a list of the various ìblocksî or sub-chapters. So, I am not alone in being unable to come up with a brief definition of mental disorder.

DEFINITION OF PSYCHIATRY We may have rather better luck in defining psychiatry.

As well as the international ICD10, there is another important psychiatric classification, which is the Diagnostic and Statistical Manual of the American Psychiatric Association, currently in its 4th revision. (Unlike the ICD, the DSM-IV, as it is referred to, is not made freely available online; subscriptions are available through the APA. Online summaries do exist, (for example), though they are unofficial and perhaps not to be relied on in a legal setting.)

Known as the DSM IV, it does at least have a long Introduction; but again there is no brief definition of mental disorder. Ultimately, the DSM describes itself merely as an account of "those matters coming to the attention of psychiatrists."

Just to say that "psychiatry is what psychiatrists do" is obviously something of a cop-out. It is clearly circular, to an almost Alice-in-Wonderland extent.

It is nevertheless probably the most realistic definition available, as it recognises that there is no unifying scientific theory as a foundation to the speciality.

VICTORY OF EMPIRICISM OVER THEORY This practical approach, placing greater emphasis on observation and description than on lofty theorising, is now in fact generally accepted in the field.

Gone are the days when psychiatrists had a faith in Freud and felt they could thereby explain almost everything about their patients and their problems.

MENTAL ILLNESS This means a mental condition (for example, an episode of depression) which has come on after a period of relatively good health and normal functioning. In other words, it has a defined beginning, albeit that this onset may be gradual rather than sudden.

Usually, there will be improvement, with or without treatment, mental illness having a strong tendency to spontaneous remission over time, though obviously not in all cases.

(Indeed, when treatment is effective, it can often be thought of as having brought forward an improvement, which might well have occurred anyway.)

Many illnesses resolve completely, and the patient goes back to the previous ("pre-morbid") state. In other cases, there will be partial improvement, ands the patient will have some residual symptoms.

LEARNING DISABILITY (lifelong low intelligence) and PERSONALITY DISORDER (permanent abnormal and counterproductive or "maladaptive" character traits, apparent by adolescence or early adulthood) are therefore distinguished from mental illness not because of the symptoms and behaviours exhibited but by the timecourse.

In fact, the clinical picture at one point of time often cannot distinguish between mental illness on the one hand and PD / LD on the other.

The difference is in the timing. In PD or LD, the patient has always been affected by them, whereas in mental illness they have had a definable beginning.

In the case of learning disability, the difficulties will be likely to be lifelong, though obviously treatment and rehabilitation may ameliorate the situation.

This applies to most patients with personality disorder also, though there may be some gradual improvement as the patient gets older.

Learning disability and personality disorder are coded in Axis II of the DSM, whilst mental illness is coded on Axis I.

TEMPORAL PATTERN Hence, clarifying the temporal pattern is of crucial importance in assessment, whether it be clinical or medicolegal. The treatment for, and prognosis of, a mental illness will probably be very different from that of a personality disorder.

But the clinical picture at any one time, say a psychiatric assessment in the clinic, is only a snapshot. It may be consistent with an illness or with a personality disorder or learning disability. Getting background information from a friend or relative and from the medical notes, is vital.

DEFINITION OF MENTAL DISORDER: LEGAL  As in psychiatry, the law does not have a universal definition of mental disorder. Rather, it defines mental disorder to suit the particular issue it is dealing with. So, there are various definitions of mental disorder in the law.

In personal injury, well-known chambers suggest that a psychiatric injury is a "..recognisable psychiatric illness.. a positive illness recognized as such by psychiatrists..".

(Again, this definition is obviously to some extent circular. But it does reflect the reality that psychiatric diagnosis is mainly a process of pattern recognition. That is, the symptoms and behaviours are assessed in relation to the possible diagnoses which might apply, and the diagnosis is then chosen which is "best fits" the observations.)

Other areas of the law define and approach mental disorder according to their own specific purpose, for example the Mental Health Act, and the new Mental Capacity Act.

And the Disability Discrimination Act is now abandoning the requirement that the mental health problems concerned must constitute a "recognized mental disorder".

But for present purposes, the proposition that "a mental disorder is present if it is diagnosed as such by a suitably qualified doctor, according to recognized diagnostic criteria" is a useful starting point.