*The national Service Framework for Mental Health: modernising mental health services. 
[Department of Health, 1998] 

According to the UK's Department of Health, ‘One adult in six suffers from one or other form of mental illness.’* Although we may be tempted to regard mental illness as little more than a form of malingering or that the suffering it involves is not as real or intense as that arising from physical illness, it is in fact no less real and no less distressing, if not more.

Meanings

Defining mental illness is difficult because, as Humpty Dumpty was trying to explain to Alice, many words do not convey the same meaning to everyone.   'Illness' and 'health, for example, are parts of a continuum; they can only be distinguished in their more extreme forms.   Less extreme, they 'shade into' each other.  
Mostly, 'illness' is used to describe the state in which people themselves, and perhaps also their families, friends, associates or employers, judge them incapable of functioning as well as they normally would or would like to; so the difficulty of establishing the exact point at which wellness becomes illness is sometimes resolved only by seeking a medical opinion.

Organic & functional mental illness

Although cases of organic mental illness are rare by comparison with the functional, many symptoms are common to both and both therefore are sometimes referred to psychiatrists for attention.   Consequently, despite the absence of evidence, some people wrongly assume that functional cases do originate from brain impairment.

Mind and behaviour

Patients suffering from functional mental illness are likely to experience emotional turmoil, irrational, jumbled or bizarre thoughts and to have difficulty making sense of what they thought they understood.   Consequently, their behaviour may go awry and perhaps be socially unacceptable.  

 In the USSR and more recently in China, political dissidents were forced to accept psychiatric treatment because their behaviour led their political masters to assume that their minds were disordered.
   
More astonishingly, 'psychopathic' behaviours, such as pyromania, kleptomania, paedophilia and sadomasochism are not normally classed as mental illnesses because no recognised medical treatment for them exists!

Malingering

Difficulty understanding the nature of functional mental illness does indeed offer opportunities for malingering, either deliberate or involuntary.    
Some people in important social positions, for example, appear to have avoided criminal prosecution because their psychiatrist certified that they were suffering from mental illness.   Apparently, too, surgeons have been known to operate, despite their professional judgement,  because they were persuaded that the patient's mental health would  otherwise deteriorate.   
Children considered to be suffering from a recognised form of mental illness, too, can easily mislead themselves or their carers to believe that their unacceptable behaviour is caused by the illness and not by them.    As Shakespeare wrote, 

Such instances might, however, be more accurately described as 'conversion hysteria' in which patients suffer genuine physical symptoms of illness but are totally unaware of their underlying mental trauma. 

Categories of functional mental illness

A more complete list of categories can be found in the American Psychiatric Association's Diagnostic and Statistical Manual  of Mental Disorders (DSM IV), currently under review, and in the World Health Organisation's International Classification of  Diseases.

Consequently, people who seek medical help for a functional mental illness are likely to be advised that they are suffer from a categorised form of the illness.   These categories are arbitrary, or ‘conjectural’, groups of symptoms, unrelated as would be the case with physical illness, to well-established causes.    
None the less, the categories enable medical practicians to select particular forms of treatment and reassure their patients that the condition is a medically recognised one.    Examples are:

agoraphobia, attention deficit/hyperactivity, anorexia nervosa, bulimia nervosa, chronic fatigue, clinical depression, irritable bowel, manic depression, multiple personality, non-organic enuresis, obsession-compulsion, paranoia, post-natal depression, post-traumatic stress, schizophrenia.  

The holistic approach

‘Stress’, a surfeit of perceptions, or 'information overload', is a major factor leading to functional mental illness.   Mostly, we are able to cope without becoming so ill that we need professional help, but our  capacity for dealing with the stress intrinsic to living is limited; an excess of entirely unexpected, perhaps unprecedented circumstances can easily overwhelm our minds.   We become ‘unbalanced and cannot ‘see the wood for the trees’.   We are not flawed or defective; simply human.
Headaches, depression, anxiety, panic attacks, listlessness, shortness of temper and nightmares may be warnings that we may need to withdraw from the situation at least temporarily in order to determine what is really troubling us and the best way of dealing with it.   

Some of these events, particularly the more traumatic, may result in personality changes.    Brain impairment is not a cause of functional mental illness, but it may sometimes be a consequence!