When Pope John Paul II arrived in Toronto this week, he once again defied expectations.

The 82-year-old pontiff, trembling from the effects of Parkinson's disease and hobbled by acute arthritis, was supposed to have been lowered from his airplane in a hydraulic lift. But the pope had other ideas.

Slowly, painstakingly, he shuffled out of the plane onto the stairway, and step by labored step he walked down onto the tarmac, aided by a cane on one side and the arm of an aide on the other.

This was John Paul's answer to those who question whether he can still function as pope.

It is obvious to all that John Paul is in the twilight of a more than two-decade papacy. He grows more frail with each day. His head droops, his hands tremble because of the Parkinson's and he cannot even hold a speech to read. He cannot turn his body around without help. He cannot finish his homilies, increasingly handing them to another cleric after reading the first few lines. He rarely walks far and is often wheeled to ceremonies in a chariot-like cart.

Yet he is in the midst of a grueling 11-day, three-country trip that his aides tried to persuade him not to take. And as he arrived in Toronto to preside over World Youth Day, a biennial celebration he started in 1985 and relishes each time it takes place, John Paul showed by walking down those stairs that he's not finished yet.

Those close to the pope say that despite his physical ailments, his mind is still sharp.

"We're in a difficult phase here," says George Weigel, the pope's biographer. "But I find him, and everybody who is with him on a regular basis finds him, still completely clear-minded, happy, looking to the future. He's going to carry this out all the way. And that he has to do that under these difficult physical circumstances is in one sense a tragedy, but in another sense a great witness."

John Paul has made several concessions to his deteriorating condition. On trips abroad early in his papacy, his frantic pace left aides and journalists in a state of chronic fatigue. Now, his schedule is pared down to the essentials. In Toronto, he spent his first three days on an island retreat, resting and recovering from jet lag. In Guatemala and Mexico, his next stops, he has scheduled only one public appearance in each country.

Even so, papal insiders were hinting that John Paul should cancel part of this current voyage. After the pontiff seemed particularly fatigued during a trip to Azerbaijan and Bulgaria, slumping in his chair and failing to utter more than a few lines of his public addresses, his spokesman, Joaquin Navarro-Valls, speculated that the pope would be able to handle only part of his North American trip. "Toronto is for sure," he told reporters, "but as for Mexico and Guatemala, we'll have to see."

The pope quickly put to rest any doubt that he would go to Latin America, where he will canonize two popular saints.

But John Paul's determination has not put to rest murmurings in the Vatican, and elsewhere, about his ability to function as pope of the 1 billion-member Roman Catholic Church. He will not visit the United States, despite the fact that the church here is struggling with perhaps its greatest crisis, with allegations of priestly sexual abuse and revelations of cover-ups by bishops.

And there is the feeling among some that John Paul, who was always content to leave the minutiae to the Vatican bureaucracy, does not have his hand on the rudder of a church in need of direction.

In recent months, some of John Paul's cardinals have uttered the unspeakable, talking of papal resignation. In May, Cardinal Joseph Ratzinger, one of John Paul's closest advisers, said the pope would most certainly resign if he ever got to the point that he could not carry out his duties.

There is no provision in Catholic church law for the removal of a pope, even one who has become incapacitated. But Pope Paul VI kept a signed letter in his desk describing the conditions under which he would relinquish his office. Pope Paul died in 1978 and the letter never came into play. There is speculation that John Paul has written a similar letter.

Church law does allow a pope to resign. The last to do so, perhaps the only pope to resign voluntarily, was Celestine V in 1294.

Before his election as pope, Celestine was first a monk and then a hermit by the name of Pietro del Murrone. He craved solitude, taking to the wilderness in Abruzzi, but his reputation as a holy man attracted hundreds of followers, who ended up establishing 36 monasteries in his name.

Pietro's life was one of harsh penance and continual prayer. "His hair-cloth was roughened with knots," according to the Catholic Encyclopedia. "A chain of iron encompassed his emaciated frame; he fasted every day except Sunday; each year he kept four Lents, passing three of them on bread and water; the entire day and a great part of the night he consecrated to prayer and labor."

It was Pietro's misfortune to be the compromise candidate in a hopelessly deadlocked conclave, in which bitterly divided cardinals tried in vain for more than two years to elect a pope after the death of Nicholas IV. After unanimously settling on the holy man of Abruzzi, a delegation ascended the mountain to inform him of his election and beseech his acceptance, which he did after shedding tears and uttering a brief prayer.

Although a saintly man, Celestine proved to be an incompetent pope. He doled out favors at will and was harshly manipulated by King Charles II, who forced him to relocate the papacy to Naples.

Celestine longed for his hermit's cell and feared that the demands of the papacy threatened his soul. In December 1294, after just five months in the papacy, Celestine abdicated.

The next pope, Boniface VIII, rescinded all of Celestine's papal acts and, fearing that the retired pope might be used by forces seeking a schism, kept him confined in the tower of a castle until his death. He was declared a saint in 1313.

Although John Paul might one day be canonized a saint, he insists he is no Celestine.

Italian journalist Vittorio Messori, who collaborated with the pope on the 1994 book, Crossing the Threshold of Hope, said John Paul recently indicated to him his intention not to retire.

"The force to continue is not my problem but that of Christ, who wanted to call me, though unworthy, to be his vicar on earth," Messori said in what he called an interpretation of the pope's thoughts. "In his mysterious design, he has brought me here. And it will be he who decides my fate."

Weigel, the papal biographer, said John Paul feels his is a mission he cannot abandon. "He does not think of himself as a corporate CEO," Weigel said. "He thinks of himself as the father of a family. And you don't resign paternity."