'The more well adapted you are to our crazy society, the crazier you will be," writes Oliver James (Sanity's shining light, December 19). Working in the NHS, I could buy that.

I'm generally on James's side for his observations on the problems of modern life, and his enthusiasm for sensible treatments such as cognitive analytic therapy. However, his attack on psychiatry and the concept of mental illness is mischievous and misleading. Psychiatry does have a legitimate part to play - a limited part, but an important one.

James moves back and forth between mental distress and mental illness as if there were no difference. Psychiatrists do not tell people "who feel like shit" that they have an illness and we can cure it. All of us will have times when we feel like that. Most of us, thank heavens, will not suffer a mental illness. There is a very real difference of degree and nature that has been recognised for centuries and in all cultures. Few who have experienced mental illness in their family fail to distinguish, for example, anorexia nervosa from food faddishness, or depression from feeling down. It did not require psychiatrists as the pharmaceutical industry's "unwitting lapdogs" to identify melancholia or schizophrenia. It is insensitive to those with mental illnesses to dismiss the severity of their suffering in this way.

Nor is it true that rates of mental illness are so much higher in affluent societies. We suspected just that in the 1960s so the World Health Organisation conducted an enormous study across nine countries specifically to test that question. They found schizophrenia (the most severe mental illness) to be about as common (and as instantly recognisable) in India and Nigeria as it is in Denmark or Britain. It simply won't wash to claim that "industrialisation and urbanisation [are] the fundamental causes of mental illness".

The general public may believe in the overwhelming influence of life experience as opposed to constitution in causing mental illnesses, but that does not mean that there isn't good evidence to the contrary. Only the most fanatical could continue to deny that genetics plays a significant part. It is, however, only part - you would have to search pretty hard to find a psychiatrist who doesn't acknowledge the influence of personal experience and environmental influences as significant causal factors.

Poverty, urbanisation and bereavement all contribute to mental illness. References to discredited theories blaming dysfunctional upbringing (the "schizophrenogenic mother", the "double bind") as "the cause" of mental illnesses are relegated to historical footnotes.

James is right that mental illnesses open challenging perspectives on tired thinking; in many ways they remind us most vividly of what it is to be human. But it is often at a terrible cost to all those involved. James's claim that emotional distress (mental illness) is a form of dissent ignores a crucial difference. Those who dissent, unlike those with a mental illness, have a choice.

· Tom Burns is professor of social psychiatry, University of Oxford, and author of Psychiatry: A Very Short Introduction
tom.burns@psych.ox.ac.uk