In the 28th canto of Dante Alighieri's 14th-century poem, The Inferno, widely regarded as one of the keystones of European civilisation, the poet imagines himself transported to the circle of hell reserved for those who sought advancement by "sowing discord".
There he encounters a man whose body, in an endless cycle of pain, is torn open by devils, heals and is again ripped asunder.
The man who is thus tormented is the Prophet Muhammad.
This scene has long been a favourite for some of Europe's greatest illustrators. Gustave Dore, William Blake, Auguste Rodin and Salvador Dali have all done their own versions of it. These images show, not just the face of the Prophet, but his naked body and even his innards. They are unquestionably, and at root intentionally, insulting. Dante drew on the medieval belief that Muhammad was originally a Christian, and that he was therefore a schismatic who deserved to have his body split in two because he had divided the one true church.
Arguably, The Inferno and the illustrations it has inspired through the centuries are much more offensive to Muslims than the cartoons in the Danish newspaper Jyllands-Posten that are currently causing so much grief.
The image of Muhammad in hell has, indeed, attracted the anger of Islamic extremists. In June 2002, there were widespread news reports of a plot by a group linked to al-Qaeda to blow up the Church of San Petronio in Bologna. It contains a 15th century fresco by Giovanni da Modena, illustrating the scene from The Inferno.
The interesting question is why it took so long for an image that is so luridly offensive to Muslims to become controversial. For centuries, of course, Muslims were unlikely to be exposed to these images, or, if they were, to have any power to object. But with the growing Islamic population in Europe and the wide reproduction of these paintings, not just in scholarly books but on the internet, there was ample opportunity for outrage a long time ago.
It didn't happen for a reason that is pertinent to the current controversy: offence has to be taken as well as given. As with the fresco in Bologna, which is tucked away in a dark side chapel, you have to go out of your way to be insulted. You have to crane your neck and strain your eyes. If you look hard enough, you'll always find what you're seeking.
Any Irish person over 40 can easily imagine the uproar there would have been here in the 1980s if a popular American show, broadcast worldwide, featured a stream of images of the Irish as violent drunks. How would we have felt about a float in a St Patrick's Day parade carrying "drunken Irish novelists" who proceed to dismount and start fights with random passers-by? Or someone being told that they're going to be shown something they've never seen before and musing, "A sober Irishman?" Or a shot of a book on a businessman's desk with the title When Irish Eyes Are Smiling, You've Just Been Robbed?
These are just a few of the anti-Irish jokes in The Simpsons - a show that is nowhere more popular than it is in Ireland. If anything, we laugh louder at these jibes than anyone else does. Twenty years ago, though, we would have been picketing the US embassy in Dublin and organising boycotts of McDonald's.
What's changed? Money, success and power thicken the skin, and ours has been so hardened by our newly confident self-image that sticks and stones feel like tickles. Offence is subjective, not objective. Mass outbreaks of high dudgeon depend on the presence of two factors. One is a group with an agenda, looking for insults to exploit. The other is a wider group of people who feel so vulnerable to denigration that their feelings are available for exploitation.
The Irish had those two factors 20 years ago. The Muslims have them now.
The current conflict is not about religion, still less about a supposed clash of civilisations.
It is about power and politics. Movements, factions and regimes are using outrage to outflank their opponents, just as they did in the 1970s over the film The Message and in the 1990s over The Satanic Verses. Their sincerity about religious insult can be measured by the way they propagate, or at least tolerate, a rancid anti-Semitism that reproduces without blushing the vile anti-Jewish propaganda of the Nazis.
For these tactics to work, though, there has to be a significant mass of people who feel threatened and humiliated enough to be hurt by images that more confident people could dismiss as puerile jottings.
However cynical the manipulation of Muslim outrage undoubtedly is, the hurt it exploits is real. It is inflicted, not by the pen of a cartoonist, but by the political realities of the last few decades. In that period two of Europe's indigenous Muslim minorities - the Bosnians and the Chechans - have been horrifically abused; the Palestinians have had their noses rubbed in degradation; the invasion of Iraq has revived a 19th-century imperial arrogance; and Muslim populations in Europe's cities have suffered from structural discrimination.
Until those underlying conditions change, Muslims will be easily provoked and Islamo-fascists will have easy pickings.