Did the Inquisition fight heretics using the same science that we now use to control disease? Michael Brooks wasn't expecting this
IT BEGAN as a light-hearted discussion over a beer, but ended in a profound revelation. Paul Ormerod, an economist at Volterra Consulting in London, was talking about his latest work on the science of networks, such as the internet and groups of friends. "That's interesting," said his friend Andrew Roach, a medieval historian at the University of Glasgow. "That sounds rather like what I'm working on." It was quite a claim for Roach to make. He studies the persecution of medieval heretics - people in Catholic countries who rejected, among other things, the unquestionable authority of the pope. But his comment sparked the pair into exploring the issue further, and it seems that Roach was right. In the 13th century, Catholic inquisitors halted the spread of heresy by exploiting insights that look remarkably similar to the science we now use to describe networks as diverse as social structures, the spread of disease and the web. The common thread is provided by what are called scale-free networks. The properties of these networks were unearthed less than five years ago by Albert-László Barabási, a professor of physics at the University of Notre Dame in Indiana, when he sent a software robot crawling around the web to analyse the links between websites. Barabási expected the bot to reveal that websites are connected to each other at random. According to a branch of mathematics called graph theory, most websites would then have roughly the same number of links. So Barabási was shocked when the bot found that lots of sites connect to just a few others, but a very small number of websites have huge numbers of links. Because no single number of links stood out, as he had expected, he called the network "scale-free". In the past few years, our understanding of scale-free networks has transformed the way we look at a startling array of physical and biological systems, from ecosystems to diseases to sexual partnerships (New Scientist, 13 April 2002, p 24). They show, for example, that a few highly connected "nodes" in the network - be it of people, computers, viruses or other biological organisms - are crucial to its operation. Without these hubs, the structure of the network falls apart. Thanks to this insight, we are now much better equipped for tasks such as fighting the spread of disease and analysing the vulnerabilities of the internet. Ormerod and Roach admit that the kind of analysis Barabási carried out can't be duplicated using medieval documents. But the similarity in character that they have now uncovered suggests that the Inquisition did, indeed, research the problem and identify what we would call a scale-free phenomenon. Heretical hubs At the start of the Inquisition, the Catholic church used a crude method of dealing with heretics. It simply instructed the crusaders to kill everyone living in villages and towns suspected of harbouring dissenters. But these were the early days, and the church hadn't realised what it was dealing with. You can't destroy a scale-free network by indiscriminate destruction. You can't, for instance, disable the internet by taking out random websites. Scale-free networks require a cunning plan. The inquisitors began to realise this when the heretics just refused to go away. The random slaughters provided some temporary respite, but heresy always revived. It's a pattern modern epidemiologists recognise: an outbreak of influenza that appears to have been eradicated will come back unless preventive measures are taken. And so the inquisitors resolved to find the best way to wipe out the heretic infection. By the 1230s, say Ormerod and Roach, the church had worked out how heresy spread, and how it might be stopped. By 1250 there were handbooks for inquisitors detailing what we now recognise as the best way to disable a scale-free network. The Dominican friar Bernard Gui, whose inquisitors' handbook is probably the best known, makes it plain that there is no point targeting an individual. All the effort should go into identifying the heretics who have visited the suspect in his or her home, as well as the guides who brought them there and escorted them away, he said. It is all about the network connections, not the nodes. Indeed, once the inquisitors had established the importance of mobility to the spread of heresy, they changed their whole approach to punishment. Penitent heretics had once been sent on pilgrimages, but by the end of the 13th century this practice had been halted. It was just too risky: the penitents could make loads of new contacts across a broad geographical area. "Attention turned instead to punishments which restricted movement or marked the penitent out, making social intercourse difficult," Ormerod and Roach point out in a paper submitted to the Journal of Social Structure. And so the custodial sentence was born: the fight against heresy was the first use of prison as a punishment in itself. Milder punishments followed the same new doctrine of isolation. Those who had knowingly mixed with heretics, for instance, were forced to wear a yellow cross on the front and back of all visible clothing. Just to be spotted associating with a cross-wearer meant risking accusations of heretical sympathy, and so this measure acted as an effective form of "inoculation" for the community. But this was still not enough to stop the spread of heresy. It was only towards the end of the 13th century that the inquisitors began to recognise the real problem. A few highly connected, highly influential and highly mobile individuals were spreading heresy faster than indiscriminate killing, imprisonment and "inoculation" could wipe it out. The inquisitors had finally realised the importance of the network's hubs. Just as the internet has, for example, Yahoo and Napster acting as short cuts to connect many people using very few links, heresy relied on the activities of a few influential people like William of Milan. If the church was to beat heresy, this well-connected heretic - and others like him - had to be stopped. In 1293 William was on the run in what is now Slovenia. The inquisitors sent out a spy to find where he was staying, then put together a task force of Franciscan friars trained in heretic hunting to capture him. According to Inquisition expense accounts held in the Vatican, the whole operation cost, in modern terms, around £25,000 to £30,000. It was money well spent. "It doesn't take an awful lot of these very well-connected characters to cause an awful lot of trouble, and the Inquisition had grasped that," says Roach. Indeed, this hitherto unrecognised grasp of the nature of heretical networks settles a few things that historians have never been able to fathom. "It explains why, for instance, when heresy was more or less dead at the end of the 13th century and there were only half-a-dozen active heretics about, everybody was still in such a tizz about it," Roach says. By this point, the Catholic authorities knew that as long as a few of the right kind of people were still active, heresy could re-establish itself at any time. Roach believes it is no accident that the Inquisition adopted the same methods that we now apply to dealing with scale-free networks: the inquisitors involved were known to think scientifically, he says. "They are mainly Dominican friars, one of the most highly educated orders. I believe a scientific process of sorts was being used." The idea that heresy networks were scale-free is "very plausible", according to Gene Stanley of Boston University, who is one of the modern pioneers of scale-free network theory. The idea that historical documents can reveal centuries-old scale-free phenomena is a welcome surprise, he says. Ormerod and Roach's work adds weight to his own claims that all social networks are scale-free. "This is fantastically original work," Stanley says. "Once you think of it, it's obvious, but that doesn't mean it's not important." Of course, the modern applications of scale-free network theory, such as disease control, are rather more palatable than the purposes of the inquisitors. So maybe Ormerod and Roach have delivered another pleasant surprise: perhaps it is a sign of humanity's progress that our understanding of scale-free networks is now saving lives, not ending them. You could almost call it enlightenment.
Michael Brooks
FURTHER READING
"The medieval Inquisition: scale-free networks and the suppression of heresy" by Paul Ormerod and Andrew Roach, www.arxiv.org/abs/co (Longer URL)