Pope Benedict has been weakening for quite some time.
At the time of his British visit in 2010 he was having to set aside hours in the course of the day for rest, and over the last year he has been visibly reduced, with meetings curtailed or cancelled.
Had other commitments not prevented it, I would have been in Rome at the end of last week as an adviser at a meeting of the Vatican's Council for Culture.
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This normally ends with a Papal audience, but even as I regretted being unable to go I also wondered whether that audience might also be cancelled.
Pope Benedict has shown himself to be a radical, or at least a realist. Some commentators said he craved the Papacy. That is false. He lamented the situation obtaining in the last part of John Paul II's pontificate when matters were left unattended or unresolved, and watched as the Pope became broken by age and illness.
Cardinal Ratzinger was already 78 and had served for one-quarter of a century in the burdensome office of Prefect of the Sacred Congregation.
He could have waited to be retired, and returned to his quiet life of prayer, writing and music, either in his Bavarian homeland or in his Rome flat where he enjoyed the company of his cats.
He knew, however, that some matters had been let slip and he also lamented the vulgarisation of religious ritual.
Catholicism for Joseph Ratzinger is not a matter of spiritual self-help, or political campaigns: it is about handing on the truth and practice that he believed descended from the early Church and included rituals and meditations on the life of Christ.
Theological, liturgical and religious order needed to be recovered and he sought to begin this task, which, given the slippage, he knew would provoke complaints. Similarly, he knew the harm done by the abuse scandals and the inevitability of the new Pope being a target for attack.
Once elected, he set about liturgical reform, writing on the life of Jesus and making leadership appointments. I doubt he thought he would live this long but the years have let him finish the first two tasks. As for the third, he was never a fighter and disliked confrontation.
His wish, I think, is for a much younger man to be elected who can carry out the internal reforms and engage in the great debate with the wider world about the meaning of human existence.
The tensions and troubles in the Vatican have accelerated his ageing, and in retrospect it is clear he has been contemplating this decision for some while.
l John Haldane is Professor of Philosophy at the University of St Andrews and Consultor to the Vatican Pontifical Council for Culture