VATICAN CITY Pope Benedict says in a new book that he would not hesitate to become the first pontiff to resign willingly in more than 700 years if he felt himself no longer able, "physically, psychologically and spiritually," to lead the church.
With startling candor, the 83-year-old Benedict floats the possibility of something Catholic Church officials do not like to talk about because it could open a doctrinal can of worms.
The book, called "Light of the World: The Pope, the Church, and the Sign of the Times," has so far made headlines for the pope's cautious opening to the use of condoms to stop AIDS.
But the book, an interview with German Catholic journalist Peter Seewald, also contains many personal reflections on Benedict's health, his daily routine and his future.
"Yes, if a pope clearly realizes that he is no longer physically, psychologically and spiritually capable of handling the duties of his office, then he has a right and, under some circumstances, also an obligation, to resign," he says.
The last pope to resign willingly was Celestine V in 1294 after reigning for only five months. Gregory XII reluctantly abdicated in 1415 to end a dispute with a rival claimant to the Holy See.
Earlier this year, when the Vatican was swept up by a new wave of sexual abuse scandals, there were calls for Benedict to step down, but he says in the book that he would not "run away" in a time of crisis.
"One can resign at a peaceful moment or when one simply cannot go on," he says.
Benedict seems to be in relatively good health but confesses in the book that he feels his forces diminishing.
"Of course, I am sometimes concerned and I wonder whether I can make it even from a purely physical point of view," he says of the demand on his strength during trips.
"The Holy Father is aware of his work and his complete dedication for the good of the Church," said newly appointed Cardinal Raul Eduardo Vela Chiriboga of Ecuador. "If something serious happens to his health it is logical that he would consider, with all the responsibility he has, what to do."
There is a provision in canon (church) law for the resignation of a pope but it has never been used. It says a pope can resign but he has do it of his own free choice and it does not have to be accepted by anyone.
But modern medicine's ability to extend life raises the possibility that a pope with Alzheimer's disease or one who suffers a stroke, for example, might not be able to decide.
Previous popes have considered resigning but the news only emerged after their deaths, whereas Benedict seems to be laying the groundwork for such a possibility so that it does not come as total surprise.
The late Pope John Paul, whose health deteriorated over the last years of his life, considered stepping down but then famously told a doctor that "there is no place in the church for an emeritus pope."
Indeed, what would an "ex-pope" do?
Church historians say a former pope would almost certainly have to disappear from the public scene, perhaps by going into seclusion in a monastery, never to be heard from again, so as not to cast a shadow over his predecessor.
They say that if a pope resigns instead of ruling for life there is a real danger that the church could slip into a schism. Some who are not in agreement with the rulings of the new pope could decide to recognize his predecessor as church leader.
Reuters has obtained an advance copy of the English version of the book, which the Vatican is presenting on Tuesday.
(Additional reporting by Antonio Denti; Editing by Mark Trevelyan)