THE decision was taken with meticulous care, but there is no reason to believe it was not taken alone, or at most with the knowledge of a tiny handful.
Cardinals present in Rome were summoned to the Sala del Concistorio, the pompous audience hall in the Apostolic Palace in the Vatican, the ceiling dripping with gilt, the marble floor reflecting the crimson and white vestments of the assembled princes of the church.
The ostensible reason was a routine Vatican duty, to hear about three ceremonies of canonisation. But Benedict XVI had a surprise up his capacious sleeve.
Sitting in a golden throne, reading calmly and quietly in Latin in the soporific alto sing-song with which Catholics around the world have become familiar over the past six years, he told them "my strengths…are no longer suited to an adequate exercise of the Petrine ministry."
He acknowledged that "prayer and suffering" were an essential part of the pontiff's ministry, but then came the intellectual meat of the announcement.
Today, he said, the nature of the pope's job had changed.
"In today's world, subject to so many rapid changes and shaken by questions of deep relevance for the life of faith…both strength of mind and body are necessary" - strength which he no longer possessed.
It was the sort of bombshell one could have expected of this pope: dry, terse, delivered with a faint tremor in his voice but without once meeting the eyes of the astonished old men arrayed before him.
No-one can doubt that it was an immensely emotional occasion for all involved, yet it passed off with the desiccated propriety of a man who has always been more comfortable with books than people.
Soon afterwards, one of the cardinals present at the announcement, Monsignor Oscar Sanchez of Mexico, summoned to Rome to hear about the canonisation of one of his co-nationals, was accosted by a journalist in the colonnade before St Peter's.
"The pope took a sheet of paper and read from it. He just said that he was resigning…All the cardinals were shocked and were just looking at one another.
Then the pope got to his feet, gave his blessing and left. It was so simple, the simplest thing imaginable. Extraordinary. Nobody expected it. Then we all left in silence. There was absolute silence…and sadness."
The total shock of the announcement was echoed in the immediate reactions to it.
Cardinal Angelo Sodano, president of the College of Cardinals and the man who will be responsible running the Conclave that elects the next pope, said it was "a bolt from the blue."
Italian prime minister Mario Monti tweeted, "I am greatly shaken by this unexpected news."
Vincent Nichols, the Catholic Archbishop of Westminster, said the announcement "shocked and surprised everyone."
Pope Benedict, 85, has been suffering from rheumatism but there is no reason to believe he is seriously ill.
And even if he had been known to be ill, that would not have made the announcement less stunning, for the simple reason that popes do not resign.
Benedict's predecessor John Paul II, for whom, as Cardinal Joseph Ratzinger, he worked for more than 20 years, suffered from Parkinson's Disease for years and his physical and mental decline were painfully visible to all, but he continued to go through the papal motions until the bitter end.
One has to go back to 1415 for the last pope who resigned in harness: Gregory XII, who was pressured to abdicate to bring an end to the Great Schism which a few years earlier had seen three rival popes in office.
Before that the closest parallel to Benedict was Celestine V, the "hopeless monk-hermit…[an] unworldly old man" in the words of Eamon Duffy elected in 1294.
"Faced with political and financial complexities which prayer and fasting seemed powerless to untangle," wrote Duffy in Saints and Sinners, "Celestine resigned after six months."
Yet despite the shortage of precedents, it is clear that Benedict has for years considered resignation a legitimate option - and probably his proximity to John Paul during his gruesome and impotent last years convinced him about it.
He once said of his boss that "if he sees that he absolutely cannot do [the job] any more, he will certainly resign." It turns out that he was not referring to John Paul, but to himself.
Pope Benedict XVI's decision to quit was of a part with the character of this very shy, private man: taken without consulting anyone except his own conscience and perhaps his private secretary, and certainly without the sort of cunning political calculation that has marked out many popes over the centuries; yet planned with the meticulousness of a very methodical mind.
Next week is the start of Lent, culminating in Holy Week and the most demanding schedule in the pope's year, one which he has struggled to fulfill in years past.
The Conclave will be held during the weeks of penance, focusing the assembled cardinals' minds on the need to come up with a new pope in time for Easter.
Despite his orthodoxy and ultra-conservatism, Benedict, the 265 pontiff, elected on 19 April 2005, has spent much of his nearly seven-year reign taking people by surprise.
His election was stunning in its own right: some of the best-informed Vatican experts didn't even put him on the short list, regarding him as too bookish, too shy and too German to have a hope.
But he seized control of the College of Cardinals, gave a brilliant if highly reactionary speech and was elected in the first round.
Ever since, he has been consistently unpredictable: attacking Islam, then praying in a mosque in Turkey, meeting victims of priestly paedophilia, touring Auschwitz, making peace with the Lefebvre ultra-conservatives - and ushering a British holocaust-denying bishop, Richard Williams, into the church, by the same act.
Liberal Catholics were deeply unhappy with him from the start, and he did nothing to appease them, but even for conservatives he was often an enigmatic figure.
But now that his decision to abdicate is beginning to sink in, Italian commentators are beginning to see some sense in it. Ezio Mauro, editor of the Roman daily La Repubblica, described it as "an eruption of modernity inside the Church"; Pier Ferdinando Casini, head of the rump Christian Democrats, said "with this gesture [the pope] has shown himself an authentic revolutionary."
Certainly the Catholic church faces far greater challenges than ever before, and Benedict seems to have realised he was no longer up to it.
The church is deeply entangled with the national politics of countries as diverse as China, Russia, Israel and Brazil, and the paedophilia scandal is still a running sore.
The "Vatileaks" scandal last year, in which a large amount of secret Vatican correspondence was leaked to the Italian media, exposed the pope as politically weak and at the mercy of his more cunning appointees.
As Pope John Paul II grew old and sick he simply delegated most aspects of the job, but Benedict could not reconcile himself to a mental abdication of this sort. He preferred simply to call it a day.
It is not hard to guess what he will do in his retirement - write more books. Benedict's close friends know how deeply he misses the scholarly life.
It is safe to assume he craves the company of his huge library and the leisure in which to cogitate, whether in an apartment a few steps from St Peter's or, as has been rumoured, in a Swiss monastery.
But his departure will present the next pope with a major problem: while Benedict lives, he will inevitably spend much of his time looking over his shoulder
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