This is for your own good" and "we know better than you" might be mantras of a totalitarian state, but they are also themes of the Mental Health Act.

There is a great deal of law which attempts to make sure that one citizen does nothing seriously to harm another. Examples include:

* the Health and Safety at Work regulations
* laws to control pollution
* inspection and safe maintenance of road vehicles.

However, where the state might have role in protecting people from their own actions, the picture is much less consistent and more controversial. It is worth remembering this when considering whether it reasonable for a particular individual to be dealt with under the Mental Health Act with a view to their own health and safety. People do not always avoid risky, harmful or dangerous activities; and the state rarely steps in to prevent such activities when they represent a risk for the participant him/herself only. The introduction and maintenance of "for your own good" laws has been controversial, for example laws relating to:

* motorcyclists having to wear helmets
* car drivers / passengers having to wear seat belts
* the use of certain "recreational" soft drugs.


Psychopathic Disorder...
... means a persistent disorder or disability of mind (whether or not including significant impairment of intelligence) which results in abnormally aggressive or seriously irresponsible conduct on the part of the person concerned.
Use of the Definitions other than Mental Illness
In broad terms, Severe Mental Impairment and Mental Impairment relate to people with learning disabilities (mental handicap) but only where this is "associated" with abnormally aggressive or seriously irresponsible conduct. Psychopathic Disorder is a definition which might be used in relation to people with learning disabilities, but can apply to others. The usefulness or otherwise of this definition is the subject of some controversy.
Attempts to Define Mental Illness
The Common Sense Approach
In the absence of any clear definition in law, a court will tend to view the term "Mental Illness" in terms of what a sensible member of the public would make of those words. In practice, it is a doctor who has to certify, when appropriate, that someone has a Mental Illness and the doctor is using a body of academic and practical knowledge in reaching her or his decision on this.

A Government Attempt
The Department of Health once published the following description of "Mental Illness":

Mental Illness means an illness having one or more of the following characteristics...

* more than temporary impairment of intellectual functions shown by a failure of memory, orientation, comprehension or learning capacity;
* more than temporary alteration of mood of such degree as to give rise to the patient having a delusional appraisal of his situation, his past or his future, or that of others or to the lack of any appraisal;
* delusional beliefs, persecutory, jealous or grandiose;
* abnormal perceptions associated with delusional misinterpretation of events;
* thinking so disordered as to prevent the patient making a reasonable appraisal of his situation or having reasonable communication with others.

Plans to beef up mental health law.

 http://news.bbc.co.uk/1/hi/health/6157736.stm