According to the UK's Department of Health, ‘One adult in six suffers from one or other form of mental illness.’ and, since its report, one UK government minister has quoted the current figure as 'one in four'.

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

Word 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 'wellness', for example, are parts of a continuum and are distinct only in their more extreme forms.   Less extreme, they 'shade into' each other.  
We use 'illness mostly to describe the state in which people – and perhaps their families, friends, associates or employers – judge themselves unable to function as well as they think they should and  the exact point at which wellness becomes illness is usually resolved only by seeking a medical opinion.

Physical illness

Physical illness is easier to determine than mental illness because it is mostly characterised by the presence of pathogens or lesions that are amenable to standard medical procedures.   Influenza, for example, results from a virus exploiting a weakened immune system; malaria results from infection by a mosquito-borne parasite and byssinosis (a lung allergy) is likely to be caused by dust in a cotton factory, and so on.

Organic & functional mental illness

The two main forms of mental illness are  'organic' and 'functional'.   Organic mental illnesses – such as the dementias that result from Alzheimer's, Pick's, Creutzfeldt-Jakob's and Huntingdon's diseases, acquired immune deficiency syndrome (AIDS) and those that result from dependence on mood-altering substances – result from brain impairment and are relatively rare.   Although called mental because they affect the mind, they could equally well be called physical because the  brain is a physical organ.    
Mental illness not caused by brain impairment is called 'functional' because it is a derangement of the patient's mind and is perhaps better described as mental disorder.  
Confusion about the different forms of mental illness tends to arise because many people omit to distinguish between brain and mind and because many symptoms of the organic and functional forms are very similar.   Consequently, both forms may be referred to psychiatrists for treatment.

Mind

Mind in fact is a compendious word that we use to describe a group of mental activities, such as awareness, perceiving, thinking, believing, wondering, remembering, calculating, analysing, judging, deciding, planning, learning, knowing, understanding, sensing, fearing, hoping, wanting, imagining and so on.   Although it can become disordered, the mind cannot be damaged or diseased in the same way as the brain, because it is not a physical organ

For example:
'To my mind ...',
'Ease of mid',
'Her mind went blank', 'A brilliant mind',
'I am troubled in mind',
 '... a thing on my mind',
'I am minded to ...',
'My mind is made up.'

Brain scan evidence misleading

Confusion may also arise from the fact that some forms of schizophrenia, for example, are thought to be organic and others functional.   Schizophrenia is a somewhat vague term, coined in 1911 by the psychiatrist Eugen Bleuler to describe the condition of people whose bizarre speech and behaviour seemed to be caused by  confused thoughts and feelings — and the word is, often applied to almost any group of patients who display similar symptoms, whether there is evidence of  brain impairment or not.  
Electronic scanning of the dynamic brain patterns of people diagnosed as suffering from schizophrenia displays some differences between their brain patterns and those of people who are not considered schizophrenic.   This has caused some researchers to assume that the condition is organic, or perhaps that it indicates a genetic predisposition.   If it is a genetic predisposition, it may – like autism or dyslexia – be more accurately described as a disability than an illness.   In either case, we have yet to discover whether the abnormalities are the cause or the effect of the condition and whether the brains of everyone diagnosed as schizophrenic are abnormal.   
Similarly, a recent Chinese study, has indicated that 17 young people suffering 'Internet Addiction Disorder' (IAD) had disrupted brain networks, whereas other 18 subjects in the study had not — another finding in which we cannot distinguish cause from effect.

.  

 .

www.bbc.co.uk/news/health-16505521

, 

Brain plasticity

The dynamic brain patterns of people in love are almost certainly different from those of people who have recently experienced, say, sudden bereavement.    Both patterns are 'abnormal' but they are symptoms of illness only in the language of poetry!
Brains are plastic.   We have increasing evidence that, if a normal function is adversely affected by brain trauma, other areas may display a capacity to re-instate the function.  
Academic surveys have revealed that the brains of London cab drivers, who had learned over a four-year period of study  all the routes, streets and places of interest within a six-mile radius of Charing Cross, had larger hippocampi compared with the ones they had before, even though their memory for other matters had not changed.   

See, for example,  http://en.wikipedia.org/
wiki/Neuroplasticity

www.bbc.co.uk/2/hi/677048.stm

www.bbc.co.uk/news/health-16086233

www.bbc.co.uk/2/hi/67704
8.stm

Perhaps schizophrenic sufferers would be better served by attempts to understand them than by examinations of the state of their brains. 

Functional mental illness categories

People who seek medical help for a functional mental illness are likely to be advised that they are suffering from a particular category of the illness — a standard medical practice reassuring them that their condition is a medically recognised one that can be cured by suitable treatment.
To that end, some psychiatrists have devised standard categories of functional mental disorders.    A few of the many  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, as well as schizophrenia.    Earlier editions of the manuals included homosexuality.  
Regrettably, however, such categories are no more than what have been called 'conjectural clusters of symptoms' because, unlike physical illness categories, they are not based on any really sound, undisputed  evidence of cause and effect.

Diagnostic and Statistical Manual of Mental Disorders(DSM IV), 2004, currently under review,.
American Psychiatric Association

 International Classification of  Diseases
 World Health Organisation 2010

Malingering?

Although we may sometimes be tempted to regard functional mental illness as little more than a form of malingering, and the suffering it involves as less real or less intense than that arising from physical illness, it is in fact no less real and no less distressing for those who suffer it.   They are likely to experience irrational, jumbled or bizarre thoughts and have difficulty making sense of what they thought they understood.   Consequently, their behaviour may go awry and be regarded as socially unacceptable.  

None the less, some cases of medically-recognised forms of functional mental illness may lead to behaviour that may seem feigned.   Children suffering from a recognised form of mental illness can easily convince themselves and perhaps their carers that their illness, not they, is the cause of their irresponsible behaviour.    Carl Gustav Jung, a Swiss psychiatrist prominent in the psychoanalytic movement, described his own experience of it as a 12-year-old child.    As one of Shakespeare's protagonists appears to have thought:

Reflections.  Pantheon, New York 1963

Hamlet  Act V, scene ii  

Similarly, there are recorded instances of people in important social positions avoiding criminal prosecution by convincing themselves and some psychiatrists that they were suffering from a form of mental illness.   Apparently, surgeons also have been known to operate despite their professional judgement,  because they were persuaded that the patient's mental health would  otherwise deteriorate. 
Political dissidents in the USSR and more recently in China, were forced to accept psychiatric treatment because the heads of state thought they must be suffering from a contagious mental disorder that could spread and thus endanger the whole community.
Yet 'psychopathic' behaviours, such as pyromania, kleptomania, paedophilia and sadomasochism are not normally classed as mental illnesses because there are so few recognised medical treatments for them!

'Social and political assumptions sometimes lie at the heart of what we regard as mental pathology ...',
Michael King, Glenn Smith & Annie Bartle, 
British Medical Journal,  21st February 2004

Managing functional mental illness:

a biological approach

A number of psychiatrists argue that, because we are essentially biological machines, the best way of treating functional mental illness is by treatments, such as chemical or mechanical intervention, similar to those used for treating organic mental illness.   The advantage of such treatments is that they provide a relatively simple, convenient and sometimes rapid, relief of the symptoms so that patients assume they have recovered and can resume their normal activities.

Disadvantages of the biological approach

Although a biological approach to organic mental illnesses is usually appropriate , a similar approach to functional mental illness is problematical.    In the absence of any evident lesion, the chances of discovering the real causes are minimal and the fact that the symptoms of the functional forms are often relieved by such methods does not demonstrate that the cause is primarily biological.   Dispelling a headache by absorbing an analgesic substance, for instance, does not indicate that a deficiency of the substance was the primary, or even a contributory, cause of the headache!
Similarly, a common belief that the hormonal imbalance concomitant with parturition is the main cause of post-natal depression does not explain why so many women give birth without becoming depressed.  
Further, certain mood-altering drugs can have long-term adverse effects on the brain, liver and lungs and electro-convulsive therapy is reputed to be even more drastic.   No one yet knows whether some of its side-effects are permanent.    

Managing functional mental illness:

a holistic approach

A different approach, sometimes termed 'holistic', is by regarding people not as biological machines, but as authentic beings willing and able – potentially at least – of determining and influencing their behaviour, albeit with the help of skilled professionals. 
But, because everyone's mind is different, no one can have exactly the same experience of a particular functional mental illness as another person and, for that reason, determining the exact causes of functional mental illness is likely to be more complex than determining the causes of physical or organic illnesses.  Functional mental illness is an existential condition that can be alleviated, but not cured, by medical treatment.   Sometimes, the symptoms of a functional mental disorder may even be an indication that the patient would like to change in a way that s/he is unaware of! 

Musar Haskei 

Stress factors

The underlying causes of functional mental illness are usually  a surfeit of perceptions or 'information overload'. 
Mostly, we are able to cope without becoming so ill that we need professional help, but our capacity for dealing with the inevitable setbacks of living is sometimes overwhelmed  by an  excess of unexpected, perhaps unprecedented, events that cause our minds to be ‘unbalanced’, unable to 'see the wood for the trees'.
Headaches, depression, anxiety, panic attacks, listlessness, shortness of temper or nightmares may be warnings that we  need to withdraw from the situation at least temporarily in the hope of find what is truly troubling us and possible ways of dealing with it that may not be immediately obvious.