From the Special Report on Death:<blockquote>As recently as a century ago, it was priests not doctors who declared a person dead. When in doubt, they looked for signs of putrefaction. As medicine advanced, however, it became apparent that death was not an event, but a process.
[F]or practical purposes an arbitrary line [has been] drawn. First it was taken as the heart stopping. Then came the notion of brain death and in the 1960’s this seemed like the way forward. For a while it was even considered foolproof: once activity ceases in the brain and brainstem you can never regain consciousness, and without intervention the body will quickly shut down.
But foolproof it is not and the fact that several hundred neurologists and philosophers are gathering next May for the fifth International Symposium on the Definition of Death shows this only too well.</blockquote>Read the full report at New Scientist.