As a student, in 1975, I attended a lecture at my local mental hospital by a dissident Soviet psychiatrist, with her artist husband present. Their manner of meeting had been most unusual: he had been referred to her for confirmation of a diagnosis of schizophrenia.

Having interviewed him, she could find nothing very loopy about his thinking. He was not suffering hallucinations and did not believe he was a poached egg (delusions). Only when he began to slag off the Marxist-Leninist system in which they lived did she begin to doubt his sanity. She had encountered that 'symptom' before, but on this occasion it suddenly occurred to her that it was nothing of the kind.

All he was saying was that they lived in a repressive regime that was badly at variance with its declared goals of bringing universal freedom and equality. In a Damascene moment she realised that any attempt to declare him mad would be purely political, not medical.

This tale chilled me for a long time afterwards, especially when I trained as a clinical psychologist: how would I be able to tell whether I was in the grip of systemic thinking, which meant I was really engaged in social control and the mislabelling of mentally healthy political dissent? How would I know if I was part of a mass collective delusion?

The capacity of groups of people to ignore what is staring them in the face was very powerfully illustrated by the lecture itself. As I was listening to the psychiatrist talking, my eye travelled to her husband, and with a gasp I noticed his hair. It was parted down the middle, with one half dark and the other completely white! To my ignorant mind, and in 1975, this seemed comically schizophrenic.

No one at all referred to this most peculiar fact in the pub afterwards, because we were all so busy pontificating about the iniquities of The Psychiatric System. Nonetheless, since RD Laing's widely read book on schizophrenia had been entitled The Divided Self, you might have supposed one of us would have commented on his hair colour.

I suspect we all wanted to ignore such an inconvenient possibility. Since those days, incarceration for being mentally ill has become much rarer. This is partly due to the much-maligned Laing, and partly due to the arrival of more effective drugs. But, above all, in the 1980s, politicians grasped that adopting a 'community treatment' approach would save a fortune. With the lunatic Mrs Thatcher in charge of the asylum, even severely mentally ill people were ejected from their expensive hospital beds.

But that does not mean my student paranoias can rest in peace. A large body of evidence now exists to call into question the established typologies for diagnosing severe mental illness. Psychologists have shown that a firm boundary between madness and sanity does not exist and that it is incorrect to suppose that manic depression and schizophrenia are as discrete illnesses as tonsillitis and flu.

· Next week: a British Marxist-Leninist goes mad.