The pontiff, the professor and 32 years of love letters
The first note is written on a small card, now slightly yellowed with age. "Dear Madame Professor," it begins, "I propose that we talk on 29th July at 8pm at 3 Franciszkanska Street in Krakow. Best regards, Card. Karol Wojtyla."
It was the start of what must surely rank as one of the most unusual relationships in the long history of the papacy.
It was 1973, and the future Pope John Paul II was the Archbishop of Krakow. His guest that summer evening was a Polish-born American philosopher called Anna-Teresa Tymienecka.
Like him, she had lived through the war years in Krakow under German occupation, but she left Poland to pursue her academic career abroad. In the United States, she married a distinguished economist and had three children. She was formidably clever, and strikingly beautiful.
She got in touch with Cardinal Wojtyla because she admired a book of philosophy he had written, and they eventually agreed to collaborate on an English-language edition.
That earned her a walk-on part in several John Paul biographies, and she might well have remained an important but modest footnote to his extraordinary life.
But she was one of nature's hoarders, and in 2008 sold her archive of their correspondence to the National Library of Poland in Warsaw, which I read in translation for the BBC's 'Panorama' programme.
The archive includes some 350 letters and notes he wrote to her, and it extends over more than three decades.
Part of its fascination lies in the way it takes one back to the Cold War; 1970s Poland was, of course, a communist state, and the Church was regarded as the enemy, so its leaders were closely watched by the secret police, known as the SB.
Marek Lasota, who runs the Institute for National Remembrance in Krakow, has turned up a file which lists everything the SB wanted to know about the city's archbishop, and the list runs right down to the type of shaving cream and toothpaste he used. "Every letter was intercepted and checked," Dr Lasota told me, "both private and official."
So when Karol Wojtyla wanted to write letters without worrying about the prying eyes of the SB, he sent them from Rome. In the autumn of 1974, he spent more than a month there at a synod of bishops, and he brought four of Anna-Teresa's letters with him.
"I didn't answer them earlier because I cannot trust our post office," he told her, "however, I have kept these letters, brought them to Rome with me and I am reading them again - they are so meaningful and deeply personal, even if they are written in a philosophical 'code'." Towards the end of the letter he adds, intriguingly: "Finally, there are issues which are too difficult for me to write about."
It seems that, at some point in the months that followed, Anna-Teresa declared her love for him, and the way Cardinal Wojtyla responded provides an important insight into his providential view of his own destiny. He believed that her presence in his life was a God-given gift, and that the relationship was a kind of vocation.
This conviction comes up again and again in his letters. In one, he told her: "Once - I remember exactly when and where - I heard these words 'I belong to you', and for me, first of all, the gift of a person resonated in them. I was afraid of this gift, but I knew from the beginning, and I know still better and better now, that I have to accept this gift as a gift from heaven."
There is no evidence whatsoever that he broke his vow of celibacy - indeed, everything in the letters suggests he kept it - but his conviction that the relationship was part of God's plan seems to have meant that he could not break it off, as many prelates of his generation would certainly have done.
As pope, Karol Wojtyla would develop a strongly providential view of his role in the collapse of communism, and he famously saw the will of God at work in the way he survived the assassination attempt by Ali Agca in 1981. It is striking to find an echo in this very personal sphere of his life.
"It's part of Wojtyla's spirituality, this tremendous sense of providence, of destiny, of the involvement of God in every aspect of his life," says Eamon Duffy, Emeritus Professor of the History of Christianity at Cambridge. "So it's not surprising that he should invest this, which is one of the most important relationships of his life, with that kind of significance."
This understanding of the relationship was reflected in an object which will seem odd and eccentric to many 21st-century readers, but which underlines the deep, old-fashioned piety which lay just beneath the surface of the Catholic Church's first celebrity pope.
Cardinal Wojtyla gave Anna-Teresa what is known as a scapular, an item of devotional clothing which is worn next to the skin.
Devotional scapulars are miniature versions of the apron-like garments which monks wear over their habits - two tiny bits of cloth hung over the chest and back - and since the Middle Ages they have been regarded as a symbol of commitment to the Christian life.
This one had been given to Karol Wojtyla by his father - who died when Karol was just 20 - and it features again and again in his letters to Anna-Teresa Tymienecka.
The letters cast new light on another sensitive area: Anna-Teresa's influence on John Paul's ideas. When he became pope, she rushed out the book they had worked on together - its English title was 'The Acting Person' - but the Vatican mounted a legal challenge.
John Cornwell, the author of 'The Pope in Winter', says: "It's clear that there were many quite important people within the Vatican who wanted her completely written out from her participation in 'The Acting Person'.
"The notion that another person had, in fact, been the author - and of all persons, a woman… - would have been quite extraordinary and unacceptable."
John Paul refused to speak up for Prof Tymienecka when she faced attacks from Vatican officials, and she felt betrayed.
Carl Bernstein, the Watergate journalist, interviewed her in the 1990s for the book, 'His Holiness'.
"What she keeps saying to me," he told me, "is they don't want my role known. They don't want to acknowledge my role, and Wojtyla, my friend the cardinal - now the Pope - has betrayed me."
The row led to a period of estrangement between them, and in the early 1980s, his correspondence consisted largely of cards sent at Christmas, Easter and on her name day.
But when John Paul was struck by four bullets in the assassination attempt of May 1981, she dropped everything to be at his side. "I am overwhelmed by sadness and anxiety, and want desperately to be close to you," she telegrammed. "I arrive on Saturday…"
She was one of the few allowed into the Gemelli Clinic to see him as he recovered from emergency surgery.
In the early 1990s, John Paul was diagnosed with Parkinson's disease, and towards the end of his life he seemed an increasingly remote figure - locked away in the Vatican and doctrinally intransigent.
But some of the most touching letters in this archive shed a rather different light on those final years, and suggest that Anna-Teresa became an important source of emotional support.
The two swapped memories in the manner of a couple with a long shared past.
After his final trip to Poland in 2002, he wrote to her of "our mutual homeland, so many places where we met, where we had conversations which were so important to us, where we experienced the beauty of God's presence".
When she described skiing trips she and her children took, he remembered mountain forays together many years earlier, recalling the thrill of the slopes even when he was almost immobilised by Parkinson's.
And she sent him pressed flowers and photographs from her country home in Vermont, where he had spent a few precious days with her family during a trip to the United States in 1976.
There is still a bit of this story that has not been told. The National Library of Poland allowed us to see John Paul's letters to Anna-Teresa, who died in 2014 - but, we discovered, they also have the letters she sent to him, which we have not been allowed to see.
However, it is nonetheless a compelling story, offering new insight into an already remarkable pope.
"We are talking about Saint John Paul," says Bernstein. "This is an extraordinary relationship," he said.
"It's not illicit - nonetheless, it's fascinating. It changes our perception of him." (© Daily Telegraph, London)