Robert Veatch weighs his words carefully when he talks about how people pass away. Most simply die. Some "become dead". Others are "made dead".

Some end-of-life cases are so unclear, he thinks, that people should be able to choose in advance the definition of death they want to be used to declare them deceased.

"Most ordinary people, including most physicians, assume whether you're dead or alive is a science question," Veatch, a Georgetown University medical ethics professor who has lectured about death and dying for over three decades, told Reuters.

"In my view, it's a philosophical and religious issue and different people have different views on the matter," he said at a bioethics seminar at Georgetown's Kennedy School of Ethics.

Thanks to medical progress, terminally ill patients or victims of severe accidents can be kept on life support far beyond the point where they would have died naturally.

Veatch asked if being permanently unconscious and dependent on feeding and hydration tubes is still really life. If not, then people taken off that support are not killed, he argued, but are "made dead" or they "become dead".

The traditional view is that death occurs when the heart and lungs stop. Since the 1970s, Western countries have defined it as the irreversible loss of the entire brain's functions.

But the brain stem can keep basic functions going - such as breathing - even in a permanent vegetative or comatose state.

So since 1973 Veatch has been advocating a third definition saying that death sets in when the higher brain functions - the thinking and feeling that make us human - are lost.

This means death comes when consciousness is permanently lost, he said: "If you've got the substratum in your brain for consciousness, you're alive. If that's gone, you're dead."

Veatch suggests the law set a default definition, most likely whole brain death, and let individuals opt out and sign a statement saying they want to be declared deceased either by cardio-respiratory death or higher brain death.

Only two places on Earth allow anything near this. The U.S. state of New Jersey lets orthodox Jews opt out of the whole brain-death idea and use cardio-respiratory death because they traditionally see breath as the key to life.

Japan uses the heart and lung criteria as a default, but lets people opt for whole brain death so they can donate organs.

"It's not an accident that we did the first heart transplant in 1968 and in 1970, we began adopting laws that change the definition of death," Veatch said. "As soon as we figured out a way to do heart transplants, we had to figure out a way to get somebody dead without their heart stopping."