The problem started some 40 years ago with the invention of ventilators - machines that keep lungs breathing and hearts pumping even after the brain has suffered extensive damage. This raised for the first time the question of whether people could or should be treated as dead simply because their brain is dead. One set of philosophers argues that the destruction of the frontal lobes, with the memories and personality they encode, is enough to declare someone dead. This definition includes those in a "permanent vegetative state" - permanently unaware, but with enough brain function left to breathe unaided. Others resist the idea of brain death altogether and insist that the heart must stop beating before a body can be treated as dead. The compromise "whole-brain-death" position, which has been written into law in most of the industrialised world, is that a person can only be declared dead if almost all brain function has been irreversibly destroyed.

Whichever definition is adopted, there are many practical and political implications. For instance, physicians are generally obliged to treat the living and to stop treating the dead. Vital organs can only be removed from someone declared dead, and brain-death laws have permitted hearts, livers and other organs to be maintained in the body by ventilators until they are removed for transplantation.

Some bioethicists, such as Robert Veatch of Georgetown University in Washington DC and Linda Emanuel of Northwestern University Medical School in Chicago, have proposed that individuals should choose their own definition of death - at some point between permanent vegetative state and the heart stopping. But having different definitions of death from bed to bed presents enormous practical and cultural difficulties. To see people treat the dead as if they were living, or the living as if they were dead, is profoundly disturbing.

As diagnostic technologies have advanced, declaring total and irreversible brain death has become ever more problematic. First, brain damage is far more complicated than the whole-brain-death model supposes. Damage to small parts of the brain can result in permanent unconsciousness, even if the rest remains intact. In the 1990s advances in electroencephalography allowed residual brain activity to be detected in many patients who would previously have been considered brain dead, and we now understand that even parts of the brain that are electrically quiet can still be healthy. Using functional magnetic resonance imaging, a team at the University of Cambridge has been able to detect near-normal brain activity in several people who had been in a vegetative state for more than six months (Science, vol 313, p 1402).

A second problem is that the growing ability to compensate for the loss of the brain's regulation of the body means that doctors can maintain some brain-dead bodies indefinitely. One argument for the whole-brain definition of death was that the bodies of those diagnosed as brain dead would soon die, regardless of medical treatment. Now some brain-dead bodies on ventilators have survived for years.

A third problem is that we have discovered more about the brain's self-repair mechanisms and are rapidly developing new ways to repair damaged brains. We now understand that the brain continues to produce neural stem cells throughout life, and drugs and gene therapies have been developed that can stimulate this self-repair capacity. It may soon be possible to engineer new neural tissue from patients' cells and transplant it into damaged areas.

Progress in nanotechnology and the miniaturisation of computing will also eventually allow brain damage to be repaired with implanted machines. Implanted wiring and computer chips already speak directly to and from brains, allowing the deaf to hear, the blind to see, and paralysed people to control computers directly with their thoughts. Implanted pacemakers can deliver electric current into the brain to suppress seizures and depression, and researchers are working on the development of mechanical implants that model and replicate the functions of different parts of the brain, such as the hippocampus.

Progress in the creation of biocompatible nanomaterials such as nanowires to conduct signals to and from neurons will increase the safety and fidelity of brain-computer implants. Eventually, if futurists such as Ray Kurzweil are right, it may be possible to release millions of nanorobots into damaged brains to determine how much of a brain-damaged person's memory and personality are recoverable, and facilitate any rebuilding.

Together these advances make it problematic to declare someone brain-dead and be certain that the conscious, self-aware person is irretrievably destroyed. Whereas some kinds of brain damage are almost certainly irreversible, others leave open the possibility that if the body and brain could just be maintained until further advances are made, brain repair could be attempted or at least a definite diagnosis made. Many people say they would not want to be kept alive in a permanent vegetative state, but few would prefer death to enduring even several years of sleep and rehabilitation.

On the other hand, there are enormous social and emotional costs involved in indefinitely maintaining the persistently unconscious in the hope of future cures. The medical and nursing costs for a permanently unconscious person in the industrialised world are many times greater than the average family income. Keeping the hearts pumping of those who have been unconscious for more than a year is a questionable use of public resources, regardless of the prevailing definition of death. As our diagnostic and remediative capabilities advance, however, it will be increasingly important that caregivers make a duly diligent effort to ascertain that the person in the damaged brain is irreversibly dead, and not just waiting for Prince Charming's kiss.

Our ability to rebuild destroyed brains may also force us to develop a new definition of death, based not on brain activity but on personal identity. To take the most extreme example, would you be the same person if your entire brain, including all your memories and personality, were destroyed and then grown anew? If we are our personality and memories, would some new person who ends up living in your brain and body still own your property and be married to your spouse?

Even the transition to an information-based view of personal identity and death, based on memory and mind rather than on the physical body, may just be the beginning of a much more fundamental shift in the way we view death. For example, when we are able to record human memory and experiences directly from neurons, we will be able to back up, edit and share our lives. If your every thought and feeling has been incorporated into the minds and experiences of others, is it really so important that you personally continue to exist?

The definers of death gathering in Cuba will still mostly be concerned with how to diagnose brain death and how brain-dead people should be treated. But the fact that there has been so little progress in defining death since the first international symposium in 1992 is making some suspect that the concept is unravelling altogether.