The Science of Discworld II

changes the rules for itself as it goes. So does a garden. As plants grow, they take nutrients from the soil, and this affects what else can grow there later. But they also rot down, adding nutrients, providing habitat for insects, grubs, hedgehogs … A mature garden has a very different dynamic from that of a new plot on a housing estate.
    Similarly, we change our own rules as we develop.
    There are always several superficially different, non-overlapping descriptions of any complex system, and one way to deal with a complex system is to collect these descriptions and choose appropriate ones for different ways of influencing its behaviour. 1 An amusingly simple example can be seen in many French and Swiss railway stations and airports: a sign that says
LOST PROPERTY
OBJETS TROUVÉS
    The French means ‘ found objects’. But we don’t think that this is a case of the English losing objects and the French finding them. It’s two descriptions of the same situation.
    Now look at a baby in a pram, throwing its rattle out on to the pavement for Mummy, or child-minder, or indeed passers-by, to retrieve. We probably think that the child is not coordinated enough yet to keep its rattle within reach: we think ‘Lost Property’. Then we see Mummy give the rattle back to the child, to be rewarded with a smile, and we think ‘No, it’s more subtle: there is a baby teaching its mother to fetch, just as we adults do with dogs’. Now we think ‘Objets Trouvés’. The baby’s smile is itself part of a complex, reciprocal system of rewards that was set up long ago in evolution. We watch babies ‘copy’ the smiles of parents – but no, it can’t be copying, because evenblind babies smile. Anyway, copying would be immensely difficult: from anywhere on the retina, the undeveloped brain must ‘sort out’ a face with a smile, then work out which of its own muscles to work to produce that effect, without a mirror. No, it’s a pre-wired reflex. Babies reflexly react to cooing sounds and to pre-wired recognition of smiles; an upwardly-curved line on a piece of paper works just as well. The ‘smile’ icon rewards the adult, who then tries hard to keep the baby doing it. The complex interactions proceed, changing both participants progressively.
    They can be analysed more easily in unusual situations, such as sighted children with ‘signing’ parents, perhaps deaf or dumb, but occasionally as part of a psychological experiment. For example in 2001 a team of Canadian researchers headed by Laura Ann Petitto studied three children, about six months old, all with perfect hearing but born to deaf parents. The parents ‘cooed over’ the babies in sign language, and the babies began to ‘babble’ sign language – that is, make a variety of random gestures with their hands – in return. The parents used an unusual and very rhythmic form of sign language, quite unlike anything they would use to adults. Similarly, adults speak to babies in a rhythmic sing-song voice, and between the ages of about six months and a year the babies’ babble takes on properties of the parents’ specific language. They are rewiring and ‘tuning’ their sense organs, in this case the cochlea, to hear that language best.
    Some scientists think that babbling sounds is just random opening and closing of the jaw, but others are convinced it is an essential stage in the learning of language. The use of special rhythms by parents, and the spontaneous ‘babbling’ with hand-movements when the parents are deaf, indicate that the second theory is closer to the mark. Petitto suggests that the use of rhythm is an ancient evolutionary trick, exploiting the natural sensitivities of the young child.
    As the child grows, its complex interaction with surrounding humans comes to produce wholly unexpected results: what we call ‘emergent’ behaviour, meaning that it is not overtly present in the behaviour of the components. Where two or more systems interact like this, we call the process a complicity . The interaction of an actor with an audiencecan build up a wholly new and unexpected relationship. The evolutionary interaction of blood-sucking insects with vertebrates paved the way for protozoan blood parasites that cause diseases like malaria and sleeping-sickness. The car-and-driver behaves differently from either alone (and