Pamela, the subject of a haunting documentary on Channel 4 tonight, developed a novel, if somewhat disquieting, mechanism to cope with her sadistic upbringing: she created new selves. When the pain, squalor and ignominy became too much to endure, Pamela, as it were, “left it all behind”: while she was abused, she dissociated and departed to another place — leaving a new person in her place.

Rémy Aquarone, an analytical psychotherapist, has dealt with these disturbing cases of what is known as Dissociative Identity Disorder (DID). “Dissociation is a primitive defence mechanism,” he said. “When something is unbearable to consciousness and cannot be cognitively processed, it is split off: quite literally dissociated.”

In many cases the various “alters” have their own memories and personality traits. When a switch is about to occur the patient often undergoes a temporary look of vacancy before the background alter “emerges”. One psychoanalyst I spoke to had worked with a patient who had a successful job in the City during the week and then travelled to the South Coast at the weekend to work as a prostitute.

One of the most fascinating aspects of witnessing such people is our own knee-jerk scepticism. I watched a tape of the documentary and found it difficult to suppress a growing sense of incredulity, as if I expected Pamela eventually to wink at the camera and say: “Gotcha!” This response is not confined to lay people. Doctors repudiated the condition when it was first diagnosed and it remains hotly contested today, regarded by many as a phenomenon that has been induced under hypnotic suggestion by over-zealous clinicians.

But why this reluctance? The problem here is not a lack of evidence — which is overwhelming — but a failure of intellectual courage. For DID strikes at the heart of the most basic myth in our intellectual vocabulary: the self.

Since we first learnt to use language we have regarded the first-person pronoun as referring to something that existed in childhood, exists today, will continue to exist in the future and — for those of a religious persuasion — will survive bodily death. We fondly think of this self as the subject of our experiences, the instigator of our actions and the custodian of our morality. We are lulled into this idea by the seeming unity of our consciousness: our various thoughts and perceptions all knitted into a seamless whole.

This cherished conception is, however, a cruel fiction. It has taken extreme cases, such as DID, to ram the truth home. Take brain dissection. In these operations, the corpus callosum — a large strand of neurons which facilitates communications between the hemispheres — is cut to stop the spread of epileptic seizures from one half of the brain to the other. Under certain laboratory conditions, two “centres of consciousness” seem to appear in patients who have had this operation.

For example, suppose that we flash the word CANNOT on a screen in front of a brain-bisected patient in such a way that the letters CAN hit one side of the retina, the letters NOT the other and we ensure that the information hitting each retina stays in one lobe and is not fed to the other. If such a patient is asked what word is being shown, the mouth will say CAN while the hand controlled by the hemisphere that does not control the mouth will write NOT. So much for the “unity” of consciousness.

What about the notion of the self as instigator of action? We naïvely suppose that we consciously decide to move, and then move. When Benjamin Libet conducted an experiment on voluntary action in 1985 he found that the brain activity began about half a second before the person was aware of deciding to act. The conscious decision came far too late to be the cause of the action, as though consciousness is a mere afterthought. Many reacted to this with astonishment. Why? Did they really suppose the body was animated by some ghostly mini me lurking behind the brain?

A more plausible theory is that which is emerging from both biology and artificial intelligence. As Daniel Dennett, the philosopher, puts it: “Complex systems can in fact function in what seems to be a thoroughly ‘purposeful and integrated’ way simply by having lots of subsystems doing their own thing without any central supervision.” The self, then, is not what it seems to be. There is no soul, no spirit, no supervisor. There is just a brain, a dull grey collection of neurons and neural pathways — going about its business. The illusion of self is merely a by-product of the brain’s organisational sophistication.

Seen in this light, DID is neither a philosophical absurdity nor a medical fantasy but a vivid demonstration of the infinite adaptability of the human mind in the quest for survival. Those who tune in tonight will feel an overwhelming sense of compassion for the pathetic figure of Pamela. But, for those who take the intellectual plunge, the most acute pity will be directed inwardly. Accepting the death of “self” is both strange and traumatic, bringing with it a profound a sense of bereavement. Except that there is nothing there to bereave.

Being Pamela, Channel 4, 9pm

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