Origin: New Scientist vol 179 issue 2408 - 16 August 2003, page 32
Author: Michael Brooks
Did the Inquisition fight heretics using the same science that we now use to
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.
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
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
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.
"The medieval Inquisition: scale-free networks and the suppression of
heresy" by Paul Ormerod and Andrew Roach
* NOTE: all emphasis is my own, the article is plain text.
I read this article after being sent an invitation to a public event (this happened around August/September, but I couldn't locate the article to post about it).
This public event was for a niche subject, that not everyone would be interested in.
The organisers asked for those who had received them to pass them on to anyone else they knew who would be interested.
I sent out over 50 invitations to individuals and notified 4 organisations who passed on the invitation to their members.
I realised when reading this article, not only was I a well connected hub, but the person who passed me the invitation originally was also a hub, and the 4 organisations I'd contacted, plus a few well-connected individuals were also hubs.
That was the first time I've been part of a Self Forming Knowledge Network without having noticed it.