Death has lost any of the simplicity it might have once possessed.

The law has traditionally defined death as heart and lung stoppage, but new life support machines can continue those functions. So the search is on for a new definition.

Leading the search is the President's Commission for the Study of Ethical Problems in Medicine and Biomedical and Behaviorial Research, which heard Friday from physicians and theologians who had their own distinct views on the matter. The commission is studying whether there should be a uniform definition of death and is required to report its findings to Congress.

The doctors leaned toward "brain death" as the correct criteria. By using this term, the doctors don't mean anyone on life support machines should be turned off, nor do they necessarily mean that anyone who is in a permanent vegetative state is dead.

They do say, however, that medical tests can determine whether a person's brain has irretrievably lost its ability to function. When that is found to have occurred, the person is dead and the scarce life support machines are being used only to "maintain a corpse," they said.

The definition at issue is the criteria to be used for declaring that someone is dead, not whether someone should be kept alive, the doctors noted.

For the theologians, it is not that simple. (Some might say that nothing is simple for theologians, of course.)

Rabbi David Bleich of Yeshiva University said the definition of death isn't merely a medical matter at all, and that attempts to define it would deny the patient and his family the right to decide whether a family member should be declared dead.

Bleich equates the use of brain death criteria with homicide, and claimed that a person remains alive as long as there is heart and lung activity. But a co-worker at Yeshiva, Rabbi Moses Tendler, said he reads Jewish law to allow a person to be declared dead when his brain has ceased functioning.

As the theological debate wore on, Catholics and Protestants also weighed in with their conflicts and fears about the use of a "brain death" criteria and the possible mistaken diagnoses and botched tests that could lead to premature declarations of death.

Now add to this mix the inevitable hypothetical that was posed to some of the witnesses before the presidential commission: A person is declared "brain dead" and is being kept alive on life-support systems; a distraught family member then stabs a dagger through the heart of the "brain dead" patient. How would a doctor testify in court as to the cause of death?

You can't kill a cadaver, one doctor replied. Another doctor replied that possibly the person could be chared with assault with intent to kill. One assumes that the rabbis would have come up with yet other answers.

The definition of death is not just a theoretical discussion at this point. Courts have already dealt with it in written opinions and various groups have proposed their own versions of a model code for defining death.

It is an important and obviously controversial issue that affects everything from medical malpractice suits to organ transplants to inheritances to political succession questions.

The debate will go on endlessly, it seems. But the need for a uniform definition is already clear and it is up to the presidential commission to decide the proper definition to be applied.

And when the bioethics panel finished with that "minor" little project, it can move on to its other mandates, such as dealing with the ethical aspects of genetic testing and research on human subjects.

Those areas are even less certain than death, as the panel will discover.