I decided to apply it to the question in hand, which was: why are so many apologies flooding out of the Vatican at present? The academic to whom my query was addressed, Dr Eamon Duffy, reader in church history and fellow of Magdalene College, should know. He has just returned from Rome where he was part of a private team of eminent historians called in to advise on the latest papal attempt to make amends.
It all began in 1992 when Pope John Paul II finally admitted the Church had been wrong 349 years earlier to silence Galileo for saying that the Earth moved round the Sun. Next he apologised for the St Bartholomew's Day Massacre, when more than 5,000 Protestants were murdered in 16th-century France. Then came contrition for the Church's role in persecuting the Jews - from the Dark Ages to the Holocaust. His International Theological Commission has completed the first draft of a 35-page apology for the Crusades. A commission has been set up to consider whether pardons should be granted to people burnt at the stake as witches in Bohemia in the 17th century. And now Dr Duffy and 40 others have begun laying the groundwork for an apology for the entire Inquisition.
This will be the big one. After all, the Church can claim that its culpability in the Holocaust was limited to acts of omission. But with the Inquisition it was, as the Pope has put it, "the sons and daughters of the Church" who turned the thumbscrews and lit the pyres.
The very term has become a byword for despotic terror and narrow cruelty - images, in the words of the poet Tennyson, of "Inquisition dogs and devildom of Spain". After the Enlightenment the words became a shorthand for what secular liberalism hated in the church - its constraint on freedom, its intellectual tyranny, its obscurantism and (in its condemnation of Galileo) its hostility to science.
Such are the verdicts of poetry and politics, but what does history say? The recent gathering of the historians of the Inquisition came some 10 months after the Vatican had opened its archives on the subject for the first time. More than 4,000 volumes had remained secret, some of them for 700 years. Eamon Duffy was one of a small team allowed to tour the 27 rooms of dusty wooden shelves, on which the crumbling, vellum-bound volumes are stacked.
"It was rather weird to pass along corridors where files from the Seventies and Eighties sat cheek by jowl with stuff from the 17th century," he said. "But, of course, we were not allowed to look at anything more recent than 1903 [when the current century's persecution of modernist Catholics was begun by Pope Pius X]."
The Inquisition was introduced as long ago as 1184, under Pope Lucius III, to track down and quash heresy. But it was in 1233 that his successor Gregory IX first appointed full-time Inquisitors, drawn from the Dominican and Franciscan orders, to track down heretics who moved swiftly across diocesan boundaries before local bishops could act against them. It was given extensive powers of judgement and could arrest suspects on the testimony of just two anonymous witnesses. In 1252 Pope Innocent IV sanctioned the use of torture to uncover "the truth". Over the centuries that followed, the Inquisition, in its medieval, Spanish and then Roman variants, resulted in thousands of Europeans dying by fire or torture.
When the opening of the archives on all this was announced, many were suspicious of the Vatican's motives - particularly after spokesmen for the Pope announced that the records showed the numbers killed to be far lower than was popularly believed. Rome presented one of the most distinguished of the researchers, Professor John Tedeschi of the University of Wisconsin, to say: "People were sent to the stake, but not in the large numbers that are bandied around."
Those, like John Cornwell of Jesus College, Cambridge, who feared that a whitewash was imminent, insisted that somewhere between 1 million and 10 million had perished in the 600-year Catholic onslaught against heretics. The Pope would select only malleable historians, he warned. And in any case two-thirds of the Vatican records had been lost when Napoleon plundered the archive in 1810.
Interestingly, most historians are impatient with these objections. Tedeschi did much of his research not at the Vatican but in Trinity College, Dublin, and concluded that much of the black legend is a calumny from Protestant propagandists. Nor can all those holding this view be dismissed as Catholic apologists. The new edition of Anglicanism's magisterial reference book, Cross's Dictionary of the Christian Church, has recently pronounced that Inquisitorial courts were, even in Spain, "more benign than secular courts of the period". And the Protestant historian William Monter, of Northwestern University, reckons that the Roman Inquisition (1541-1590) killed 1,235 people across Europe, compared with 1,500 executed for heresy by the secular authorities in the preceding 20 years. Some 2,000 more died at the hands of the Spanish Inquisition.
"What the church did was repellent," said Dr Duffy, "but then so was the behaviour of authority in general." He cautions against the tendency to dehistoricise the Inquisition and see it from the standpoint of our own time.
At the time, church and state were so closely linked that heresy was seen as a form of treason. The impact of the Inquisition was determined by the interests of secular authority. The government in Venice curbed it; the monarchy in Spain exploited it to seize the lands of Jews and Muslims in an epidemic of ethnic cleansing.
"It was an emperor, Frederick II, [rather than a cleric], who `routinised' the penalty of burning people alive. In England it was Queen Mary who ordered the most concentrated spate of persecution in 16th-century Europe - the bishop charged with it dragged his feet and was reprimanded by the Queen in a letter which he put on display at St Paul's Cross in an attempt to pass the blame and show he was only obeying orders," said Dr Duffy. "Some 72 Protestants were killed by the Inquisition in Italy throughout the whole of 16th century, but 273 were burned in three years by Mary Tudor."
The Inquisition proper never really took in England. Clerics and laity alike there had become too attached to common-law procedures such as trial by jury to be happy introducing the practices of Roman imperial law on which canon law is based. The Inquisition's secrecy, its eagerness to accept denunciations, the absence of counsel for the accused, the lack of any right to confront hostile witnesses, all seemed alien. The two French inquisitors sent to conduct the proceedings against the Knights Templar in 1309 complained bitterly of the lack of qualified torturers in England.
Yet even if all this is not on the scale that received wisdom dictates, it is still bad enough. Why has Rome waited until now to consider apologising? The answer was given to the assembled historians by the Pope's personal theologian, Fr George Cottier. The year 2000, which ushers in the new millennium, should be a year of jubilee. The tradition of jubilee began in ancient Israel. It was a year when the earth was left fallow, debts were cancelled and slaves were set free. It was a time to be reconciled. Throughout its history the Church has maintained this tradition in small ways, but a new millennium must be a Great Jubilee and in preparation there must be a great repentance "of past errors and instances of infidelity, inconsistency and slowness to act".
But Dr Duffy and the other eminent historians of the period were only the first step. "After we had left the Vatican the dozen theologians who had listened as we presented our papers met again, on their own, to decide what to recommend," he said.
To an outsider it may seem obvious. Before the historical experts departed, one of them, the Jewish historian Carlo Ginsburg, made a devastating intervention from the floor, in which he suggested that any talk of asking for pardon for the past was unreal: the dead could not forgive. The Pope and the Church should rather say they are ashamed of the past without asking easy absolution.
Things are more tricky than that for the pontiff, however, thanks to the concept of infallibility. This has two aspects. The commonly understood one is that the Pope claims he speaks infallibly when he pronounces, ex cathedra, some definitive doctrine pertaining to faith or morals. But the Church also claims there is an infallibility which is revealed through what it calls "the ordinary magisterium", the collective wisdom of its bishops.
The conundrum, therefore, is this: how could the Church have been wrong in teaching for 1,500 years that "error has no rights"? It is a particular poser for Pope John Paul II, for he was a key player in drafting the Second Vatican Council's 1965 Declaration on Religious Liberty which overturned the old teaching: the demand for freedom of conscience had been a precious resource for him in his confrontation with Polish communism.
Those, like Archbishop Lefebvre, who could not accept that 1,000 years of consistent Catholic teaching had been mistaken, were so outraged that they broke with Rome. So how can the Pope now admit Rome was wrong on the Inquisition without further fracturing its authority?
One of the Pope's men had a go, at the conference in Rome. Fr Jean-Miguel Garrigues traced the emergence of the Church's bullying mentality back to St Augustine. The fatal moment came, he said, when the great fourth- century theologian misconstrued a phrase from the Gospel parable of the wedding feast and decided that the words "compel them to come in" legitimised the use of force in religion. This Augustinian approach then entered politics, with disastrous results.
But Garrigues went on to make a distinction which the Pope had already hinted at. Significantly, John Paul II has spoken not of the need for the Church to repent its errors but for "the sons and daughters of the church" to do so, as if the Institution itself bore no blame. Garrigues then suggested that the Inquisition was the result of the personal moral failure of individuals. The doctrinal magisterium had been silent on the issue, thus the Church's authority was unscathed.
Eamon Duffy and the other historians were unimpressed. Consider the weight of the history - a succession of inquisitions, under several popes; a bull on witchcraft giving papal endorsement to a ragbag of superstitions that caused countless thousands of harmless or eccentric women to be burnt alive; an index of forbidden books; a uniform practice of repression and censorship at the heart of the Church's own governance; and its endorsement of a particular relationship between church and state.
"All this surely constitutes more than silence," said Dr Duffy, rather scathingly. "It can hardly be treated as the incidental activity of some of the Church's children." Occam's razor might suggest a simpler explanation. "The bald fact is that Roman theology has simply not yet found a way of admitting that the institutional Church itself can err and sin."
There was one man noticeable by his absence at the conference. There was no sign of Cardinal Joseph Ratzinger, the prefect for the Congregation for the Doctrine of the Faith - as the body which was once the Inquisition is now known.
This latter-day Grand Inquisitor may not have recourse to the thumbscrews but his present-day interrogation of Catholic dissidents still carries echoes of the old ways. Hearings are in camera. Accusers remain anonymous. There is no defence council. The accused is initially not even told the specific charges against him, and cannot call witnesses in his defence.
Until all that changes, it may well be that any apology for the terrors of the Inquisition will have rather a hollow ring.