Church law obviously provides clear guidelines about how to proceed after a pope's death, and there are even a few precedents -- albeit ancient ones -- about how to proceed if the pontiff decides to retire. But there has never been a case in which the leader of the world's 550 million Catholics was judged physically or mentally unfit to continue to act as pope.
"There are a great many problems with the whole idea of an incapacitated pontiff," said the Rev. Alistair Sear, a church historian. "Who judges if he is incapacitated or not? Should the normal steps of papal succession take place? What happens if a pope judged to be incapacitated recovers?"
Most Vatican insiders are reluctant to talk about this issue except in general theoretical terms. They insist that despite the pontiff's frail appearance, increasingly ravaged by Parkinson's disease, the longest serving pope in a century remains extremely alert and mentally engaged.
Vatican sources believe that contingency plans for just such an emergency may be under discussion behind closed doors. Some believe that John Paul already may have signed a document outlining a plan that would go into effect in the event of a stroke or some other serious deterioration of his condition.
The most crucial decision is how to determine that the pope's health has eroded to the point that he can no longer be effective. Criteria do exist for ailing bishops, according to scholars, and they could be applied to the pope, who is first and foremost, the bishop of Rome. The rules say, for example, that when the bishop can no longer communicate, he must retire.
One person who could be called upon to make that determination could be the dean of the College of Cardinals, who as the chamberlain -- or camerlengo -- of the Vatican is the official who declares the pope's death and then takes over as the administrator of the Vatican until the conclave elects a new pope.
But according to Vatican scholar Andrea Rossi, any senior church official could make that determination.
There are those who say that by any logical definition the long-serving pope already is incapacitated and that he has been for some years.
His once-powerful voice is often all but incomprehensible except to those who are extremely familiar with it, and even with help, he no longer can take more than a few steps. Doctors say his puffy face and frequent inability to stay awake could be a sign of powerful drugs and steroids that the pontiff takes to prevent his condition from worsening even further.
"You have someone who is probably already being propped up by those behind him to some degree. If that is the case there is virtually no limit to how long it can continue given the advances of modern science," said one long-time Vatican watcher, speaking on condition of anonymity. "Centuries ago, science was incapable of that."
Given the powers of modern science, it opens another possibility: that the pope could decide to retire voluntarily, a development that the Vatican is only slightly more prepared for than incapacitation.
Catholic bishops are required to offer their resignation once they reach 75, and cardinals stop being eligible to take part in the conclave that elects the pope. But John Paul has shown no inclination to make use of this provision.
The pope is in good company in seeming to want to remain firmly on the throne of St. Peter. Aside from the 14th and 15th centuries, when as many as four men claimed the papacy simultaneously, the last time a pope voluntarily stepped down was in 1294, when 85-year-old Celestine V resigned after less than four months in office, and then lived his final two years in luxurious captivity under his successor, who feared that, if he were free, the former pope could divide the loyalty of the faithful.
Furthermore, Celestine V was demonized after he stepped down. The worst treatment came 50 years later when Dante placed him among the souls suffering in hell in the Inferno section of his epic poem "The Divine Comedy."
"Obviously, though the precedents for resignation exist, they are not very encouraging," Rossi said.
John Paul, who became pope in 1978, already is the sixth-longest-serving pope ever and the man who has ruled over the world's Roman Catholics for the longest period since Leo XIII, who reigned from 1878 to 1903.