Benedict XVI resignation: The two-pope problem
The Pope has resigned because he felt he was no longer up to the demands the office made on him.
That hasn't happened in 600 years.
In 1294 the hermit Pietro da Morrone, elevated to the papacy with the title of Celestine V because the cardinals couldn't agree on anyone else, felt likewise after only six months in the job, and gave up.
He wanted to return to his hermitage, but Boniface VIII, his successor, thought it wiser to lock him up in a convenient castle for the rest of his life, fearing he might become a rallying-point for the disaffected.
And, as it turned out, there was no shortage of disaffection during Boniface's pontificate.
One of the arguments marshalled by Boniface's many enemies was that, because popes could not resign, he wasn't the legitimate heir to St Peter.
Electing an antipope?
That may have been a long time ago but the same arguments are beginning to appear.
Two distinguished Italian theologians have called on Benedict XVI to withdraw his resignation, one arguing he ought not to resign, the other claiming a pope cannot resign.
In the latter case, when the cardinals proceed to elect a successor they are, according to Enrico Maria Radaelli, electing an antipope, an impostor on the chair of St Peter.
There are those in the Church who well might exploit such ambiguities were the new pontiff to choose a very different path from that of his predecessor on, for example, the role of women in the Church or - rather less contentious - the promotion of the traditional Latin liturgy.
And outside the Church, a schismatic group called the Society of St Pius X has long been on the verge of declaring a "sede vacante", of claiming that the pope was not a legitimate successor to St Peter because he had accepted the teachings - as they do not - of the Second Vatican Council of the early 1960s.
Benedict laboured hard to bring them back into the Catholic Church, far too hard in the eyes of some, but finally failed.
The collapse of negotiations occurred not long before Benedict announced his resignation, and may have contributed to his sense of weariness.
The Society of Pius X remains a separate Church, attracting disaffected Catholics, yet another division within Christianity.
Very oddly, the Vatican, in conjunction with Benedict himself, has decided to make matters unnecessarily complicated.
A (very) short lesson in Catholic theology is necessary here: the rank of deacon, priest and bishop is regarded as sacramental, on a par with baptism or marriage. A bishop can resign his job, as can a priest, but theologically, says the Church, they are still bishops or priests.
But being pope, however, is an office, it is not a sacramental status.
The pope is pope because he is bishop of Rome. He can stop being bishop of Rome (all other bishops are expected to submit their resignation at 75), and therefore can stop being pope.
No problem there.
Many people expected that Papa Ratzinger would revert back to being Cardinal Ratzinger, which is what happened to two rival popes in 1415.
Instead of that sensible solution, it has been announced he will be "Pontiff emeritus", dress in white and be called "Your Holiness", hopelessly muddying the waters and making him appear a quasi, alternative pope.
What about Georg?
The confusion gets worse. Benedict is keeping his private secretary, Archbishop Georg Gaenswein.
But Archbishop Gaenswein is also currently head of the papal household and therefore papal gatekeeper, though it is likely the incoming pontiff will make a new appointment. Benedict will continue to live in the Vatican.
The new pope may well find such proximity uncomfortable, feeling obliged to consult him especially on subjects Benedict made his own, the creation of the "Ordinariat" to receive Romeward-bound Anglicans, for example, or the controversial reintroduction of Latin into the Catholic liturgy.
Benedict has indicated that he will henceforth live in seclusion: he was always happier with his books (and cats) than with people, so that should not be too much of a burden.
He intends to write. He was always insistent, as Pope, that his theological writings came from the pen of Joseph Ratzinger rather than that of Benedict XVI, though one can't help feeling the office helped the sales.
And perhaps there is no realistic alternative to the Mater Ecclesiae convent in the Vatican grounds for his residence.
Were Benedict to leave the security of the Vatican City and return, say, to his beloved Regensburg, some might attempt to sue him with failing to handle properly the clerical abuse cases which came before him, while others might turn his residence into a shrine, a rallying place for dissent from any new departure by the incoming pontiff.
But there are legitimate questions about his title of pontiff emeritus. It opens him to accusations of pride, when he has hitherto been widely praised for his humility.
Michael Walsh is a papal historian and author of several books about the Papacy, including The Popes: 50 celebrated occupants of the throne of St Peter