The Science of Discworld II

haunted (in a very material sense) the markets in the East End of London in the 1950s. Prince Monolulu would accost pregnant women with the cry ‘I will tell you the sex of your baby, money back guarantee!’ Many ladies fell for this ploy, and paid a shilling, then about a fiftieth of one week’s wages.
    Level One of the trick is that random guesses would guarantee the Prince 50 per cent of the money, but he was much more cunning than that. He improved the scheme to Level Two by writing the prophecy on a note, putting it into an envelope, and getting the sucker to sign across the seal. When it turned out that the anticipated John was reallyJoan, or Joan was John, the few who bothered to return to reclaim their money found that, on opening their envelope, it contained a correct prediction. They didn’t get their money back, because Prince Monolulu insisted that what was in the envelope was what he had originally told them; the sucker must have remembered it wrong. In reality, the envelope always contained the opposite prediction to the verbal one.
    History is a complex system; its entities are people, its rules of interaction are the complicated ways in which human beings behave towards each other. We don’t know enough sociology to write down effective rules at this entity level. But even if we did, the system-level phenomena, and the system-level rules that govern them, would almost certainly be emergent properties. So the rule that propagates the state of the entire system one step into the future is not something we can write down. It is an emergent dynamic.
    When the system-level dynamic is emergent, then even the system itself does not ‘know’ where it is going. The only way to find out is to let the system run and see what happens. You have to allow the system to make up its own future as it goes along. In principle only one future is possible, but there is no short cut that lets you predict what will happen before the system itself gets there and we all find out. This behaviour is typical of complex systems with emergent dynamics. In particular, it is typical of human history and of biological evolution. And cats.
    Biologists learned long ago not to trust evolutionary explanations in which the evolving organisms ‘knew’ what they were trying to achieve. Explanations like ‘the elephant evolved a long trunk in order to suck up water without bending down’. The objectionable item here is not the reason why the elephant’s trunk is long (though, of course, that can be debated): it is the phrase ‘in order to’. This endows elephants with evolutionary prescience, and suggests (wrongly) that they can somehow choose the direction in which they evolve. All this is obvious nonsense, so it’s not sensible to have a theory that attributes purpose to elephant evolution.
    Unfortunately, a dynamic looks remarkably like purpose. If elephantevolution follows a dynamic, then it looks as if the end result is predetermined, in which case the system ‘knows’ in advance what it ought to be doing. The individual elephants need not be conscious of their objective, but the system in some sense has to be. That would be a good argument against a dynamic description if the evolutionary dynamic for elephants was something we could prescribe ahead of time. However, if that dynamic is emergent, then the system itself, along with the elephants, can find out where it’s headed only by going there and discovering where it gets to.
    The same goes for history. Being able to put a name to a historical period only after it’s happened looks remarkably like what you’d observe if there is a historical dynamic, but it is emergent.
    This far into the discussion, it may seem that an emergent dynamic is no better than no dynamic at all. Our task now is to convince you that this is not so. The reason is that although an emergent dynamic cannot be deduced, in complete logical detail, from entity-level rules, it is still a dynamic . It has its own patterns and regularities, and it may be possible to work with those directly.
    Exactly this is going on when a historian says something like ‘Croesus the Unprepared was a rich but weak king who never maintained a sufficiently large army. It was therefore inevitable that his kingdom would be overrun by the neighbouring Pictogoths, and his treasury would be plundered’. This kind of story proposes a system-level rule, a