In the former Soviet Union, opposition to Marxist-Leninism was frequently regarded as a symptom of schizophrenia by psychiatrists. How sure can shrinks in Britain today be that they are not engaged in a similar exercise?

When I was working in a mental hospital, I met Julia, a woman who had completed a postgraduate degree and published a Marxist-feminist critique of the American electoral system, before dropping out of academia to pursue left-wing, feminist politics. In her early thirties, she was anxious and mildly depressed, but nothing approaching the hallucinations, delusions or paranoia of schizophrenia.

One day she was sitting at the kitchen table about to share a pot of tea with other patients, when she said, 'Don't drink it, it's poisoned.' We ignored her with the fluid skill that groups have in persuading themselves that such comments do not exist, but this proved to be the first note in a symphony of symptoms that became a hideous cacophony over the next fortnight.

She believed there were two transmitters at either end of the city that were sending a signal through the house, driving her mad. She passed hours crouched naked in the bath, scrubbing her clothes 'to get them clean'. A fear of 'dirty', poisoned food precluded eating.

At times, she was dazzlingly insightful; like the last minutes of a dying light bulb, these insights illuminated a great deal but they were followed by days of psychotic darkness. Eventually, she was moved to a locked ward for her own safety.

I am absolutely confident that Julia's hostility towards capitalism and the patriarchy was not regarded as a symptom of mental illness by any of the professionals treating her. The truth was that her loathing of The System in general and of men in particular came from having been badly maltreated by her father.

Whenever she got close to either, she began to talk in a frenzied way about their 'contradictions', dragging her knowledge of Marxist theory into an explanation of why a man had jilted her or why a policeman should not have stopped me for speeding on the way to the hospital.

Her understanding of Marxism's application to British society was in direct proportion to the difficulties she was having. As they increased, she was forced to apply her defensive Marxism in greater detail and in ever-widening contexts, with increasing brilliance. In the end it was Julia's contradictions, and not those of the capitalist system, which led to a collapse of her capacity to cope.

I agreed with a lot of what she said about Britain, but where there is much more room for discomfort, specifically about our mental health system, is the schema with which psychiatrists, the medical establishment who control the care of the mentally ill, go about defining and treating people like Julia.

· Next week: the medical model of mental illness.