There is nothing, anywhere, to "prove" that Bernadette Soubirous saw anything at all. The best we have is the image of a sickly, lice-ridden 14-year-old child, kneeling in ecstasy in a malodorous wilderness called Massabieille, half a mile from a benighted mountain town buried in the Pyrenean foothills.
Covered in blood and pig hair, the site of the apparitions was of such low repute that the ultimate put-down for a poorly-reared local was that "he must have been educated at the banks of Massabieille".
As vehicles for divine intervention, Bernadette and Massabieille presented an unlikely package. "If God wanted to perform a miracle, he wouldn't use a little thief", was the first response of the police captain who assumed that her tale was a cover-up for the theft of firewood. Her father, as we say nowadays, was already "known" to the gendarmes.
Not long before, he had been arrested on suspicion of stealing grain. Drink, a casual attitude towards the business of the flour mill they operated and brutal luck, had left the Soubirous and their four children in terrible straits.
Their lodging by now was a dark, stinking, vermin-infested room in an old jail and Bernadette had become a maid-of-all-work and shepherdess to a woman who had nursed her as a child. It was the ultimate humiliation for a young girl who as the eldest of the family, was born to certain respect and expectations under the Pyrenean system of inheritance. Now, she was reduced to jobs normally destined only for younger siblings, a blow to her expectations that even today, Pyreneans acknowledge as central to her psychology.
Far from being the humble shepherdess at the heart of a gentle, pious, rural idyll, as often portrayed, she lived a life that, even by the grim, near-famine standards of the time and region, was a constant hardship.
Probably already sick with the tuberculosis that eventually killed her, she also had to endure psychological torment at the hands of the woman who had once been like a mother to her.
It was just three weeks after her sympathetic father helped her to return home - from one hell-hole to another - that she had her first apparition. "The visions occurred, therefore", writes Ruth Harris, "at a time of a rupture, within weeks of coming back to her family, as well as to humiliating and miserable poverty."
Put like that, it all sounds rather opportune.
But that would be to do Harris (and Bernadette) a disservice. In a book that sings with integrity, exhaustive research and a rare concern for context and suffering humanity, the Oxford fellow and tutor in modern history set out simply to write a work of history, "to counter forgetfulness about the shrine's origins" with all its elements of shame and heroism.
But for this secular Jew, then suffering from a condition for which medicine had no diagnosis and only intermittent treatment, her journey into the Lourdes phenomenon also became part of a personal voyage, an act of sympathy with 19th-century pilgrims: "Like us, they lived in an age that extolled the value of scientific rationality, and yet they knew its limitations all too well; in response they remained loyal to a spiritual world that championed the miraculous in spite of secular ridicule".
For all its benighted poverty in the 1850s - and it bore startling parallels with Ireland, with its disdain for the laws passed in a distant capital, mass emigration to South America, disease, dispossession, indebtedness and famine conditions caused by a 40 per cent increase in the population and subdivision of land - Bernadette's Lourdes was no spiritual backwater.
Straddling some of the great pilgrimage routes to Rocamadour, Montserrat and Compostela, its populace had been reared on a steady diet of pilgrim stories from the Middle Ages, tales that marvelled at the Virgin's beneficence and the possibility of miracles, with the rich as ready as the poor to entertain the possibility of the magical and the legendary, featuring small white fairies and lurking demons, and special trees and fountains which acted as magical intermediaries between the population and Christian saints and in which the Virgin Mary's appearance was associated with certain flowers and healing bushes.
All over the Pyrenees between the 13th and 17th centuries, shepherds and shepherdesses claimed to have had direct contact with the Virgin, through visions or the miraculous discovery of images, and love for her was at the heart of Pyrenean religious life. In just one 25-kilometre stretch near Pau, there were no fewer than 40 shrines to which entire parishes would descend, barefoot or on their knees, to make supplication during local and regional pilgrimages.
Especially significant in the Lourdes context were the nearby pilgrimage sites of Betharram - where the altar depicted the Virgin appearing to shepherds and where Bernadette reportedly bought her treasured rosary - and Garaison, where in the 16th century, in a story strikingly similar to Bernadette's, an innocent, ignorant, pious 12-year-old shepherd girl, communed with the Virgin (who also smacked her at one point).
A central question for Harris, then, was not why ordinary people were prepared to believe in apparitions but why, in a place and time alive with visionaries, Lourdes and Bernadette, against all the odds, became the chosen ones. Why was it her story that would rise above that of all the other young local seers - of whom one at least, had a claim as compelling as Bernadette's? How did this particular shrine continue to grow, to the point where it now attracts five million pilgrims a year? At first glance, it seems obvious: the new railway line installed a few years later; the defiance of the ordinary people goaded into defending the shrine by bullying anti-clerical authorities.
But in 1858, the omens were terrible. Bernadette's own mortified mother smacked her for her early accounts of the apparitions and the school's scornful mother superior tersely ordered her to stop the pranks.
Then the authorities got rough, chief among them being the Police Commissioner who apparently insulted her and tried to repress the shrine before it even got going. Nor was the Church - keen on its prerogatives, wary of attack and sceptical of the "superstitious" - especially pleased to begin with. The attributes that later made her so attractive - the poverty, simplicity, ignorance - were precisely what the clergy and the educated held against her then.
Even the apparition itself was a rank disappointment; a gentle, joyful white girl - not the more familiar Madonna - who relayed the most minimal of messages, telling the unfortunate child three secrets which she could never reveal. As for the important words the apparition did utter, "I am the Immaculate Conception", they were decidedly discordant to the pious ear. It was as if she said that she was beauty rather than that she was beautiful. It all seemed very unsatisfactory.
Ultimately, however, what separated Bernadette from the rest were those very words. For, as Harris makes clear, the story of Lourdes in those early decades is a story about France, about the struggles of Catholics in the aftermath of revolutionary turmoil, the capacity of the Second Empire to adjust to and even profit from, religious movements, and the inability of the Third Republic to suppress them.
Only four years before, in the wake of the revolutions of 1848, when forced to flee radical rebellion at home, Pope Pius IX had enunciated the dogma of the Immaculate Conception, thereby confronting the onward march of secularism with a defiant assertion of faith in the miraculous. This was received with great enthusiasm by the who seemed like the way it created an even greater chasm between Catholics on the one hand and Protestants and freethinkers on the other. So, to a beleaguered Pope, who had declared himself a prisoner in the Vatican in protest at losing his temporal power and property in a newly re-unified Italy, the timing of the Lourdes apparitions and its message must have seemed truly miraculous.
And thus did the apparition's statement bring together the disparate worlds of rural piety and the Vatican.
That the apparitions occurred in France was also significant for the Church, as the country was seen as the seed-bed of anti-religious sentiment and the home of the revolutionary tradition and policies that had dispossessed it of property, prestige and influence. Despite this bitter legacy, there were signs of regeneration, above all in the burgeoning Marian piety of the period which became the "age of Mary". Before and after Lourdes, a steady stream of notable apparitions was reported, including the visions of Catherine Laboure in Paris which produced the miraculous medal. Significantly, they were also seen in Pellevoisin in 1876, where they enjoined the faithful to wear a scapular with the sign of the Sacred Heart, the symbol of the Bourbon dynasty, and were also taken up by monarchists disappointed by the failure of their campaign for Restoration.
It was an alliance of these forces that saw the national pilgrimage to Lourdes become a vital way for Catholics to show their solidarity and strength. Shaped by the Paris-based Assumption Order as part of an overall campaign aimed at nothing less than restoring the Bourbons, releasing the Pope from his Vatican "prison " and re-establishing the alliance between throne and altar, it in turn, had a profound impact on the women of Notre-Dame de Salut, a lay charitable organisation of dogged and devoted society ladies, who raised the funds to transport the sick to the Grotto and travelled as carers. The Assumptionists also brought their influence to bear on the Petites-Soeurs de l'Assomption, the nuns who nursed the sick and who celebrated an ethos of selfless sacrifice and sisterly piety that would make them famous throughout France.
Lourdes is unique, says Harris, because of this special mission and because of the women who shaped it. And it was a mission of true heroism, devoted as it often was to bandaging the suppurating wounds of putrescent, decaying bodies in the days before antibiotics and steroids. Lourdes made the disgusting elevating and brought the hidden into the open. And if only for a brief phase every year, turned French society upside down.
But parts of the movement also mired it in shame. Alongside illustrations of miraculous cures, gentle nuns and chivalric men, the highly political leader writers of Assumptionist publications, which reached over half a million weekly readers, spouted a river of venom towards Jews, freemasons, and republicans.
For the Assumptionist leadership and many in the Lourdes Hospitalites of this period, anti-Semitism was an integral part of their piety, the dark side of their veneration for the Virgin and the Eucharist, just as much later, Lourdes would come to symbolise the partnership between the Catholic Church and the new Vichy regime in the second World War.
Nonetheless, the legions of miracules continued to grow. In 1897, to celebrate 25 years of national pilgrimage, 18 trains left Paris and other cities with more than 30,000 pilgrims, around 1,000 of whom were malades. As usual, they arrived in the white trains. But another, painted in the papal colours, was reserved for more than 300 miracules, whose presence immeasurably heightened the emotion of the crowd. Despite the colour, splendour and explosive emotion of this extraordinary procession as it moved through the sick and dying, the Assumptionist leader, Pere Picard, wanted still more. With his cry : "Now, invalids, if you have faith, arise!" more than 40 malades arose and walked towards the priest.
It was the kind of occurrence that doubtless brought out the worst in writers such as Zola (whose deceitful novel about the shrine became a best-seller) or the cynical Parisian medical establishment, led by the neurologist, Charcot, which already scornfully dismissed the mostly-female miracules as hysterics (a catchall phrase that explained everything and nothing) and deplored the glacial baths as infectious cess-pits. But the fact remained that even by the most exacting medical standards, Lourdes was producing inexplicable cures.
Its Medical Bureau was led by no-nonsense doctors, acutely aware of the power of suggestion and placebo. As evidence mounted in favour of something beyond the natural - one such cure involved the instantaneous fusion of two bone fragments - the cynics' arguments became increasingly convoluted, not quite discounting the usefulness of Lourdes - indeed, Charcot and other had taken to referring patients there on occasion - but placing the source of cure with the "subliminal unconscious". Those who took the intermediate stance, says Harris, eventually became central to the developing notions of the unconscious and the discipline of psychoanalysis. Meanwhile, it is whispered, miraculous events still take place at Lourdes. In most cases, the beneficiaries refuse to talk about it and who could blame them?
So where lies the truth? Is it possible to be truly objective about such a phenomenon? Ruth Harris, who travelled as a worker/pilgrim, writes that although Lourdes did not convert her, it touched her deeply. (She did not find a cure there for her complaint, which she says has since alleviated.)
As an a la carte Catholic and the planet's most reluctant pilgrim when first hauled along to Lourdes and other shrines, I am unable to explain my own relationship with the shrine. Ever since that first visit, I have been splashing myself and loved ones with Lourdes water, and while in France, have taken lengthy diversions to visit the sanctuary. Fatima and Knock - which share the same sense of packaged piety, rampant consumerism and mass mobilisation of crowds - have failed to move me in the same way.
I don't participate in the rituals in Lourdes nor have I the generosity of body and spirit to spend holidays caring for the sick there, as some souls do, where to this day, the core of its mission is what Harris describes as "the intense physicality, backbreaking labour, and centrality of pain and suffering".
So why do I go? Because at the Grotto, I experience a kind of spiritual embrace like nowhere else. I cannot explain it nor do I try. To know that it is there is enough.
Lourdes: Body and Spirit in the Secular Age by Ruth Harris is published by Penguin at £25 in UK