The Science of Discworld IV

become more sceptical as adults, and that too many adults fail to do so, hence, alas, astrologers, mediums, priests and the like.
    We can see just how indiscriminately juveniles pick up information through something that happened to Jack. He ran an extramural class in animal-handling for about thirty years, and became very impressed by the distribution of animal phobias (although he did realise that this was a very peculiar group of students in that respect). About a quarter of the students had a spider phobia, rather fewer had snake phobia (which, if bad, included worms). Some had a phobia for rats and mice. A few reacted badly to birds, feathers orbats. It seems likely (but we can’t document it in this instance) that these phobias came about by cultural infection: Mother screamed when she found a spider in the bath, or a television series depicted snakes as poisonous. (Less than 3% actually are, but it might be wise to assume lethality as a default, for solid evolutionary reasons.) Rats are often depicted as being dirty, and the same goes for mice. Jack never worked out what gave rise to phobias about birds and feathers, but it certainly passes on in families, and it’s much more likely to be learned rather than genetic. It might be a great example of how beliefs can pass from brain to brain like a computer virus, in this case not transmitted verbally. But we can see how useful these phobias would have been when we were much nearer to nature. They let us learn what creatures to avoid, instantly. And while it didn’t much matter if we occasionally avoided an animal that was actually harmless, the same mistake the other way round could be disastrous.
    Beliefs are formed through interactions between an individual’s brain and his or her environment, especially other people but also the natural world (spiders!) So it’s worth taking a general look at interactions.
    If A acts on B, we call this an action; but if B also (re)acts on A, we say that A and B are
inter
acting. A baby and its mother are like that. But most interactions are not just some sort of exchange, and they have a deeper effect: A and B are, to a greater or lesser extent,
changed
by the interaction. They then become A' and B'; then they interact again, and again, and are changed still more. After several changes of this kind, A and B have become quite different systems.
    For example, the actor walks out onto the stage, and the audience reacts; the actor reacts to this, and the audience in turn reacts to the actor’s new persona … and so on. In
The Collapse of Chaos
we called this deeper kind of interaction ‘complicity’, giving a familiar word a technical meaning that is not too far removed from the usual one, but also hinting at a mix of complexity andsimplicity. The complicity between child and mother, later between child and teachers, then with sports teams, then with the whole adult world, is the Make-a-Human-Being kit we talked of earlier. We also need a word for this cultural interaction, and have suggested ‘extelligence’. Individuals are
in
telligent; there are useful ideas and abilities somehow represented, remembered and readied for use, inside their brains. But most of a culture’s collective knowledge is outside any given individual, forming a body of information that is not in any one brain, but
outside
; hence
ex
telligence. Before the invention of writing, most of a culture’s extelligence resided in the entire collective of brains, but when writing came along, some of it – often the most important to the culture – didn’t need a brain to contain it; only to extract and interpret it. Printing boosted the role of this type of extelligence, and modern technology has led to its dominance.
    Where do our beliefs come from? From complicity between our intelligence and the extelligence that surrounds it. This process continues into adulthood, but its greatest effect occurs when we are children. St Francis Xavier, co-founder of the Jesuits and a missionary, is quoted as saying ‘Give me the child until he is seven and I’ll give you the man’. A trawl of today’s premier extelligence, the internet, will haul up an almost endless range of interpretations of that phrase, from benign to malign, but their common ingredient is the malleability of human intelligence at an early age, and its fixity thereafter.
    Until fairly recently, almost all people were religious believers. The majority still are, but the proportions depend