VATICAN CITY - Pope Benedict XVI seems worn out.

People who have spent time with him recently say they found him weaker than they had ever seen him, seemingly too tired to engage with what they were saying. He no longer meets individually with visiting bishops. A few weeks ago he started using a moving platform to spare himself the long walk down St. Peter's Basilica.

Benedict turns 85 in the new year, so a slowdown is only natural. Given his age and continued rigorous work schedule, it's remarkable he does as much as he does. This month he confirmed he would travel to Mexico and Cuba next spring.

But a decline has been noted as Benedict prepares for next weekend's grueling Christmas celebrations, which kick off two weeks of intense public appearances. And that raises questions about the future of the papacy given that Benedict himself has said popes should resign if they can't do the job.

The Rev. Federico Lombardi, a Vatican spokesman, has said that no medical condition prompted the decision to use the moving platform in St. Peter's and that the platform is merely designed to spare the pontiff the fatigue of the 100-yard walk to and from the main altar.

Benedict rallied during his three-day trip to Benin in West Africa last month, braving temperatures of 90 degrees and high humidity to deliver a strong message about the future of the Catholic Church in Africa.

Wiping sweat from his brow, he kissed babies who were handed up to him, delivered a tough speech on the need for Africa's political leaders to clean up their act, and visited one of the continent's most important seminaries.

Back at home, however, it seems the daily grind of being pope - the audiences with visiting heads of state, the weekly public catechism lessons, the sessions with visiting bishops - has taken its toll. A spark is gone. Benedict does not elaborate off-the-cuff much anymore, and some days he just seems wiped out.

Take his recent visit to Assisi, where he traveled by train with dozens of religious leaders from around the world for a daylong peace pilgrimage. For anyone participating it was a tough, long day; for the aging pope it was even more so.

"Indeed I was struck by what appeared to me as the decline in Benedict's strength and health over the last half year," said Rabbi David Rosen, who had a place of honor next to the pope at the Assisi event as head of interfaith relations at the American Jewish Committee.

"He looks thinner and weaker . . . which made the effort he put into the Assisi shindig with the extraordinary degree of personal attention to the attendees (especially the next day in Rome) all the more remarkable," Rosen wrote in an e-mail.

That Benedict is tired would be a perfectly normal diagnosis for an 84-year-old, even someone with no known health ailments. He has acknowledged having suffered a hemorrhagic stroke in 1991 that temporarily affected his vision. And his older brother, who has a pacemaker for an irregular heartbeat, has expressed concern about Benedict's own heart.

But Benedict is not a normal 84-year-old, both in what he is called to do and the implications if he were to stop.

Popes are allowed to resign; church law specifies only that the resignation be "freely made and properly manifested."

Only a handful have done so, however. The last one was Pope Gregory XII, who stepped down in 1415 in a deal to end the Great Western Schism among competing papal claimants.

There's good reason others have not followed suit: Might the existence of two popes - even when one has stepped down - lead to divisions and instability in the church? Might a new resignation precedent lead to pressures on future popes to quit at the slightest hint of infirmity?

Yet Benedict himself raised the possibility of resigning if he were simply too old or sick to continue, when he was interviewed for the book Light of the World, which was released in November 2010.

"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," Benedict said.

The former Cardinal Joseph Ratzinger watched as Pope John Paul II, with whom he had worked closely for nearly a quarter-century, suffered through the debilitating end of his papacy. After John Paul's death at age 84, it was revealed that he had written a letter of resignation to be invoked if he became terminally ill or incapable of carrying out his duties.

And it should be recalled that at the time Benedict was elected pope at age 78 - already the oldest pope elected in nearly 300 years - he had been planning to retire as the Vatican's chief orthodoxy watchdog to spend his final years writing in the "peace and quiet" of his native Bavaria.