So they think you are dead ... but are you?

How do we know when we are really dead? It's easy enough if you're the Pope. Before they elect a new one, the papal chamberlain, the cardinal camerlengo, hits the dead pope on the forehead with a silver hammer to make sure he is dead. After all, you would not want to elect a new one and then find the old one had come round and you had two.

The case of the epileptic Daphne Banks - the farmer's wife who "died" on New Year's Eve and then was found to be still alive in the mortuary as 1996 began - is, thankfully, a rare one. But the fact that, despite all the technology of modern medicine, it is possible for a living person to end up on a mortuary slab touches fears that lodge deep in the human psyche. And more than that. It raises some profound questions about the nature of death, how we define it and how we react to our own definitions.

Premature burial is a fear that has haunted and mesmerised men for aeons. And it usually is men. When Edgar Allan Poe, who was obsessed by the prospect, wrote a story on the theme there was an outbreak of panic in which a celebrated carpenter of the times made a fortune from a special coffin in which the occupant, should he recover consciousness, could signal to the quick above with bells and flags.

The nightmare, however, is of much longer standing. In the first century the magician Simon Magus, according to one report, buried himself alive, expecting a miracle - which failed to materialise. On Iona, in the sixth century, one of St Columba's monks, Oran, was dug up the day after his burial and found to be alive; when he told his fellows that he had seen heaven and hell, legend has it, he was promptly dispatched and re-interred on the grounds of heresy. And the 13th-century Thomas a Kempis, the reputed author of the great devotional work The Imitation of Christ, was never made a saint because, it was said, when they dug up his body for the ossuary they found scratch marks on the lid of his coffin and concluded that he was not reconciled to his lot. Thankfully for Mrs Banks, it never came to that.

When it comes to life and death, religion is supposed to have some answers. But the traditional definition of death as the moment when the soul leaves the body does not get us very far.

"It's not a meaningful definition," says Rev Prof John Bowker, author of The Meanings of Death. "Theologically, the soul is characteristically what the human being is when it is fulfilling its functions and purposes unimpeded and in the fullest possible extent. To say 'when the soul leaves the body' - while it's an attractive image, and did much for the banalities of religious art - really doesn't mean much."

Doctors are, of course, more precise. The trouble is that they keep changing their definitions. "Death is a process rather than an event and it can be defined at a number of points in the process," says Dr Bryan Jennett, emeritus professor of neurosurgery at Glasgow University and the man who did the pioneering work on defining Persistent Vegetative State (PVS). "In biblical times it was when breathing stopped. Then the stethoscope was invented and it became when the heart stopped. Now with modern methods of monitoring brain activity it is defined as brain death."

The modern definition also hedges its bets. "It is the irreversible loss of the capacity for consciousness, combined with the irreversible loss of the capacity to breathe spontaneously and therefore to maintain a spontaneous heart beat," according to Dr Christopher Pallis of Hammersmith Hospital, who is the author of The ABC of Brainstem Death. "It's usually straightforward to determine, but drug intoxication may cause a reversible disturbance in brainstem function. So may hypothermia. And in the past when there were epidemics of cholera and typhoid, people got very dehydrated and had an unrecordable blood pressure and were thought to be dead. That did happen."

What exactly happened to Mrs Banks is unclear. Both family and medics have remained closed about it. In modern medicine, with its negligence suits and compensation payments, there is more at stake than academic precision or ethical brow-furrowing.

So do philosophers have anything to contribute? Their approach has been to shrug the whole business off. To them the two key questions are: what is death and why does it matter? Epicurus was uncharacteristically stoical on this: "Where death is I am not; where I am death is not; so we never meet", was the broad thrust of his riposte. Others such as Heidegger have got all semantic and wondered does death have the same meaning to us when we use it of ourselves as it does when we speak of others? But most of them have shuffled the mortal coil off on to the shoulders of cultural analysts and sociologists.

So we have Ivan Illich railing against the "medicalisation of death". The art of dying has given way to "guaranteed terminal care". Doctors once saw it as their job to withdraw once the patient had entered the atrium of death; Galenic tradition prided itself on discerning when the threshold had been crossed at which nature itself broke the healing contract and the healer had to acknowledge his limits.

But with the advent of aggressive medical technology, doctors have switched emphasis. They now concentrate on the fight against death in which the patient has become the mere battleground. Today, as Illich sees it, the medical establishment has assumed the functions of a church - "shaping people's beliefs and perceptions, needs and claims in a post-Christian liturgy that instils a keen fear of pain, disability and death". All this foils the dying person's willingness to accept the inevitable. Yet the wise person today, as in the time of the ancients, needs to acquire an amicus mortis.

Two decades after Illich's Medical Nemesis was published, some doctors have come to agree. "One mother said to me: 'My son died at the roadside but the funeral was six years later'," recalls Prof Jennett of one of his PVS patients. "You have to recognise that there comes a point when treatment is futile and you have to cease. The hospice movement has done a lot to change attitudes."

But not all changes have been for the best. Legal battles over "the right to die" in recent years have highlighted a shift in our attitudes to what death is.

Today we do not simply think of death in terms of brain activity but in particular kinds of brain activity. "What we seem to value is consciousness," says Pat Walsh of the Centre for Medical Law and Ethics at Kings' College, London, "and we seem to have a scale of consciousness - which is why foetuses, the mentally handicapped, and those in persistent vegetative state are thought to have fewer rights. The less autonomous people are, the less we value them.

"What I find alarming is this move to say that when there aren't higher forms of consciousness we can count those people as dead." In Italy, doctors are already using PVS patients for organ donation. And in the US medics have begun to talk about taking organs from anencephalic children, who are born with part of their brain not developed. "They will die anyway but they can breathe, make noises and show signs of being able to pick out their mother's voices," says Pat Walsh. "What we are seeing in such cases is that in our society the concept of what it is to be dead is being redefined." And unlike Mrs Banks, such individuals may never get a second chance.