The Catholic Church sees itself as historically rooted, and while there are protocols for how the new pope will be chosen in a few weeks, history offers little guide in the case of a pope who decides to resign.
Pope Celestine V's abdication in 1294 was "the only one in papal history," writes John Julius Norwich in 2011 in Absolute Monarchs: A History of the Papacy.
Other historians may disagree but Celestine's biographer, Jon Sweeney, concurs.
"The only case in history is Celestine V in 1294, where a pope willingly and on his own accord resigned the papacy," the author of The Pope Who Quit: A True Medieval Tale of Mystery, Death, and Salvation, told CBC News by telephone from Evanston, Ill.
The resignation of Gregory XII in 1415 is often cited as the last abdication by a pope, but this is misleading and does not apply as a precedent for Benedict.
Gregory was one of three competing popes in the last years of the four decades-long leadership dispute within the church hierarchy known as the Western Schism. To end the split, the Council of Constance ordered the resignations of all three. Both Gregory and John XXIII agreed to resign, but another Pope Benedict, the thirteenth, refused and was then excommunicated.
(More than five centuries later, Angelo Giuseppe Roncalli also took the name John XXIII when he became pope in 1958.)
According to canon law
Sweeney says that the current Pope Benedict "resigned exactly in the way canon says you are supposed to do it." That canon law has its origins in Celestine's resignation.
"Because of the Pope’s position as the supreme authority in the Church, Celestine declared that the Pope could freely resign, that it was permissible and that, because, as supreme authority, it did not have to be accepted by anyone," historian Donald Prudlo said in an interview with Vatican Radio.
"It just had to be freely manifested," he added, using the language of the current Code of Canon Law. That's what Benedict did before the cardinals on Monday.
Sweeney notes that the way Benedict resigned was "shockingly similar" to how Celestine resigned. They both read a statement to a gathering of cardinals and others, although Celestine was in Naples at the time. "It was a very similar statement," Sweeney adds.
In his resignation statement, Benedict only said that "With regard to myself, I wish to also devotedly serve the Holy Church of God in the future through a life dedicated to prayer."
Vatican sources have offered tidbits of information about that future. Vatican spokesperson Federico Lombardi said that Benedict "will move to the Papal residence in Castel Gandolfo [24 km south of Rome] when his resignation shall become effective." Once renovations to the monastery of cloistered nuns inside the Vatican are completed, Benedict will live there.
That will be quite different to what happened to Celestine after he resigned. Celestine's successor, Boniface VIII, who had both advocated for and manipulated Celestine to resign, "immediately realized that it's not in his interest to have another pope still alive and walking around, so he had him hunted down and imprisoned in a castle, where he died two years later," Sweeney says.
Celestine's desire had been to return to his earlier life as a hermit.
Since Celestine did not get the chance to have a public life after stepping down, there's little precedent there to guide the Vatican today. Lombardi did say that Benedict will not join the cardinals in choosing the next pope, most of whom he appointed. The rules state that a cardinal has to be under 80 years old to take part in the conclave, and Benedict is 85.
Before becoming pope, Benedict was Cardinal Josef Ratzinger. Vatican-watchers debate whether he will return to using that title or will retain the "Pope" honorific in some form.
"We lack a law, so far, on the status of a former pope, of someone who resigned the papacy," Msgr. David-Maria Jaeger, a professor of canon law at Rome’s Pontifical University Antonianum, told Vatican Radio.
"It is possible either Benedict XVI in the next few days, or his successor, will make such a law," Jaeger added, noting that "we are in uncharted waters."
Yiftach Fehige, an associate professor at St. Michael's College at the University of Toronto, tells CBC News that Benedict, "remains a retired archbishop with the title of cardinal, and will live a life accordingly."
After Benedict's successor is chosen, Sweeney expects "we will never hear from him again."
"I think he's going to retire to Bavaria or to a monastery, and he's going to be just very quietly out of the way, and I can't imagine we will see any more of him. Otherwise, it will get very confusing and I would imagine it will become troublesome."
A surprise and not a surprise
Benedict's announcement took the world by surprise, but a year ago this week, Sweeney wrote a column for The Huffington Post under the headline, "Would Pope Benedict XVI Ever Quit?"
When asked by CBC News, Sweeney said the column was merely a product of what he was seeing and hearing at the time.
As John Paul II 's right-hand man, Benedict has witnessed up-close the physical decline of John Paul II in his last years as pontiff. After he took over the papacy, Benedict had clearly stated, in 2010, that a pope could resign if he "clearly realizes that he is no longer physically, psychologically, and spiritually capable of handling the duties of his office."
In his resignation letter, Benedict says that to be pope, "both strength of mind and body are necessary, strength which in the last few months, has deteriorated in me to the extent that I have had to recognize my incapacity to adequately fulfill the ministry entrusted to me."
Brian Flanagan, an American theology professor, argues in his blog that the precedent that Benedict has now set "may well be his greatest gift to the church." It opens the door to future papal resignations before physical incapacity sets in, and he also says "it helps restore our understanding of the papacy to that of an office rather than a personal possession."
In The Pope Who Quit, Sweeney writes about how in 2009, Benedict had aligned himself with Celestine's "memory and legacy." During a visit to L'Aquila, Italy, following the devastating earthquake that had hit the area, "Without explanation the pope paused for several minutes, removed the pallium from around his shoulders and laid it gently on Celestine's glass-encased tomb," Sweeney writes.
The Y-shaped pallium is a symbol of the pope's authority.
Both Celestine and Benedict had professed reluctance to become pope. Sweeney says Celestine's papacy "was a disaster and it was a mess, so it made perfect sense that he stepped down."
It's too soon to say what Benedict's legacy will be, but Sweeney notes that he really just continued John Paul's papacy.
"History books will look at the two of them in tandem."