Twenty years ago, it was almost unheard of; now it's an epidemic. Is Multiple Personality Disorder an illness of our own making? Bryan Appleyard reports
Multiple Personality Disorder (MPD) is a condition in which a patient appears to split into two or more separate personalities. These different personalities may or may not know of the existence of the others. Some patients may acquire just one "alter" - the clinical name given to each personality - or they may acquire dozens, even hundreds. In extreme cases one person can contain an entire population of different people, arguing, demanding, some even subverting and abusing the activities of the others.

The archetypal multiple was "Sybil" whose story was told in a "multibiography" published in 1973 and subsequently in a film. An intelligent career woman, she found she was losing periods of time and coming to in strange places with no knowledge of how she got there. A therapist uncovered 16 separate personalities - some of them children, some men. Prior to Sybil the few known cases involved only two, three or four alters. Some were aggressive: those that knew about the others argued, fought and tried to destroy them. Sybil provided the perfect case history of MPD, even down to the ascribed cause - sexual abuse by her mother in childhood - and her fame helped to create the modern awareness of the disorder. She dramatised the suffering and chaos of the multiple's life.

It is an extraordinary phenomenon, one that seems to raise deep questions about consciousness, about what it is to be human. The very thought of multiple personality induces a kind of vertigo, a self-referential spin: what would it be like to be split into hundreds of different people? Are we, without realising it, already split? After all, the lost time experienced by Sybil happens to all of us occasionally and simple moodiness can seem to change people's personalities. It all makes a kind of sense.

But the really strange thing about MPD is that it is new and it is localised. It didn't exist in the early 19th century, and even as late as 1972 it was said that fewer than a dozen cases had been reported in the preceding 50 years. But a decade later there was talk of an epidemic. By 1986, it was estimated that 6,000 cases had been diagnosed in North America and, by 1992, every town of any size had hundreds of multiples undergoing treatment. This "epidemic" is centred on North America. Most other countries have either no cases or a trivial number.

MPD has led to bizarre legal conflicts. Murderers have pleaded innocence on the grounds that the crime was committed by an alter and, therefore, they could not be held responsible. A woman is currently charging her husband with rape because he had sex with an alter and so she cannot have consented. Given our vulnerability to American fashions, we can probably expect such weirdness to start happening here.

But before it does, we should think carefully about what is going on. There are two distinct responses to this epidemic: firm belief or hard scepticism. The firm believer will say that MPD has always existed, but only now and only in certain places is it being properly diagnosed. The hard sceptic will say that this is a case of fashionable hysteria, a copycat epidemic in which patients and therapists collude to spread a disease that does not, in fact, exist. Unhappy, disordered people are simply seizing on MPD as a convenient explanation for their misery.

After the first flush of excitement about the disorder - stimulated by films and books - the sceptics have gained ground. MPD has widely been associated with child abuse and patients do usually remember incidents of abuse. But the obsession with child abuse has resulted in some high- profile disasters. The therapeutic recovery of supposedly "buried" memories of abuse became wildly fashionable in America to the point where it seemed that almost everybody had been abused. But it soon became clear that often therapists and social workers, consciously or unconsciously, were planting these memories in vulnerable patients. Equally, the sensational claims of "ritual" abuse - involving Satanism - were largely discredited by the revelation that there was almost never any firm evidence to support the claims. The hostile reaction against the child abuse obsession also tended to discredit MPD as a distinct condition. They were too closely related. Some psychiatrists began to say that MPD was a clinical invention.

Nevertheless, child abuse happens and some people's personalities do appear to split. It may be an invention, but it still happens. For MPD agnostics, the problem appears intractable.

Now Ian Hacking, Professor of Philosophy at the University of Toronto, has come up with a new answer, or rather a new way of thinking about the problem. In a brilliant book entitled Rewriting the Soul: Multiple Personality and the Sciences of Memory, he analyses the phenomenon as a specifically contemporary way of dealing with the Western conception of the soul.

Professor Hacking accepts the existence of MPD. He does not believe with the hard sceptics that it can be dismissed merely because it is local and new. He insists that all disorders have a symptom language. A disordered person will find ways of describing his - or, in the case of MPD, more often her - unhappiness that fit in with current interpretations. This does not make their disorder any less "real".

To many, this will be shocking. It appears to undermine the solidity of medical knowledge. If disease turns out to be such a slippery concept, so dependent on our ability to describe it, then it seems we can know nothing at all, only the transient, culturally determined expressions of pain and disorder that our language allows us. We are left with only a floating, postmodernist world of mere appearance. Or put more bluntly: did cancer not exist before we acquired the vocabulary to describe it?

Professor Hacking is no easy postmodernist. His point is much more profound and has implications for all the human sciences. Knowledge of humans does not work in the same way as knowledge of stars, plants or animals. There is an interaction between the knower - doctor or therapist - and the known - the patient. However scrupulous the knower might be in avoiding placing suggestions in the patient's mind, there will always be a context, a language in which both will locate their descriptions of the disorder.

The context of MPD and of child abuse - Professor Hacking describes the former as a parasite and the latter as a host - is memory. Memory became an object of knowledge in the 1870s in France. After the Franco-Prussian war there were many head injuries, and doctors became curious about the effects on personality. Simultaneously, child abuse also became an object of knowledge, it became a category distinct from mere cruelty. The modern sciences of memory were born.

This is a crucial moment in cultural and scientific history because it represents the point at which science turned its attention inwards to bring the human personality into its realm. Practitioners such as Jean Martin Charcot, the French neurologist, and, later, Sigmund Freud began to define human personality in terms of memory. What you remembered or forgot became what you really were. The sciences of memory began to supplant the idea of the soul as the key discourse of the modern self. Marcel Proust's novel A la Recherche du Temps Perdu represented the greatest artistic flowering of this French discovery of memory as the centre of human life.

Throughout most of his book, Professor Hacking avoids any evaluative language. His point is not to say whether this is a good or bad thing, nor even to say whether one type of description is more right than another. He is solely concerned to show that understanding ourselves is directly related to what ways of understanding are available. We tell stories about ourselves and the stories change. We used to tell stories about ourselves that invoked the notion of our soul; now we explain ourselves with the idea of memory.

The knower and known are linked by a feedback loop which is constantly making adjustments to the world and to the past. We actually change the past with our interpretations. So, for example, a father who is excessively intimate with his daughter in 1800 might not at the time be a child abuser. But from the perspective of 1995, he is. What "really" happened must take account of our changing awareness of intention, motivation and acceptability. No camcorder in the sky could ever fix history, private or public, as an objective narrative beyond reinterpretation.

This is a necessarily brief summary of an argument that even in the 270 pages of Professor Hacking's book seems compressed. It may not, in this summary, be persuasive. You must read the book. For I believe the importance of Professor Hacking's work can scarcely be understated.

First it subverts the bitter, partisan debates about MPD and child abuse by placing the entire issue in a new light. Second, it offers a new and more human understanding of the way we talk about ourselves, of the kinds of stories we tell to make sense of our existence. And, third, it demonstrates the power of good philosophy at a time when scientistic barbarians such as the geneticist Steve Jones can seriously claim that philosophy is to science what pornography is to sex.

This last point is, perhaps, the biggest. Professor Hacking shows that philosophical imagination and analysis has much to teach science. His withering analysis of the incompetent uses of statistics in psychology should be required reading for every over-confident science student. Scientists currently need philosophers more than philosophers need scientists.

And, finally, though Professor Hacking goes to enormous lengths not to make judgements, he climbs off the fence in his final chapter. He wishes neither to be a hard sceptic nor a hard believer. But, clearly, most of what he says inclines towards the sceptical position. He admitted to me that, if he had written the book five years ago when the recovered memory craze was at its height, it would have been more of a polemic against phony or incompetent therapists.

But the position he seems to accept in his moving final chapter is that of the "cautious and tentative" sceptic. Such sceptics who are not philosophers, he says, will honestly inquire about the reality of MPD and will be forced to accept doubts. The philosopher, however, can tell them, has an obligation to tell them, that, in multiple therapy, some kind of false consciousness is involved, some kind of obstruction to true self-knowledge.

And here, at last, the cautiously reflective philosopher comes out of his shell with the assertion that multiple therapy is contrary to what the philosophers call freedom. It is contrary to our best vision of what it is to be a human being. It is good to have philosophy back where it belongs - in real life, at the heart of things. And it is good to know that, if the MPD epidemic does land in Britain, the cautious, humane sceptics will be prepared with some formidable intellectual backing.

'Rewriting the Soul: Multiple Personality and the Sciences of Memory' by Ian Hacking is published by Princeton University Press.