The Science of Discworld II - The Globe

2. THE UMPTY-UMPTH ELEMENT

Discworld runs on magic, Roundworld runs on rules, and even though magic needs rules and some people think rules are magical, they are quite different things. At least, in the absence of wizardly interference. This was the main scientific message of our last book, The Science of Discworld. There we charted the history of the universe from the Big Bang through to the creation of the Earth and the evolution of a not especially promising species of ape. The story ended with a final fast-forward to the collapse of the space elevator by which a mysterious race

(which could not possibly have been those apes, who were only interested in sex and mucking about) had escaped from the planet. They had left the Earth because a planet is altogether too dangerous a place to live, and had headed out into the galaxy in search of safety and a long-term chance of a decent pint.

The Discworld wizards never found out who the builders of the space elevator on Roundworld were. We know that they were us, the descendants of those apes, who'd brought sex and mucking about to high levels of sophistication. The wizards missed that bit, although, to be fair, the Earth had been in existence for over four billion years, and apes and humans were present only for a tiny percentage of that time. If the entire history of the universe were compressed to one day, we would have been present for the final 20 seconds.

Quite a lot of interesting things happened on Roundworld while the wizards were skipping ahead, and now, in this present book, the wizards are going to find out what those things were.

And of course they're going to interfere, and inadvertently create the world we live in today, just as their interference in the Roundworld Project inadvertently created our entire universe. It has to work like that, doesn't it? That's how the story goes.

Seen from outside, as it sits in Rincewind's office, the entire human universe is a small sphere.

Large quantities of magic went into its manufacture and, paradoxically, into maintaining its most interesting feature. Which is this: Roundworld is the only place on Discworld where magic does not work. A strong magical field protects it from the thaumic energies that surge around it. Inside Roundworld, things don't happen because people want them to or because they make a good narrative: they happen because the rules of the universe, the so-called 'laws of nature', make them happen.

At least, that was a reasonable way to describe things ... until human beings evolved. At that point, something very strange happened to Roundworld. It began, in various ways, to resemble Discworld. The apes acquired minds, and their minds started to interfere with the normal running of the universe. Things started to happen because human minds wanted them to. Suddenly the laws of nature, which up to that point had been blind, mindless rules, were infused with purpose and intention. Things started to happen for a reason, and among these things that happened was reasoning itself. Yet this dramatic change took place without the slightest violation of the same rules that had, up to that point, made the universe a place without purpose. Which, on the level of the rules, it still is.

This seems like a paradox. The main content of our scientific commentary, interleaved between successive episodes of a Discworld story, will be to resolve that paradox: how did Mind (capital

'M' for 'metaphysical') come into being on this planet? How did a Mindless universe make up its own Mind? How can we reconcile human free will (or its semblance) with the inevitability of natural law? What is the relation between the 'inner world' of the mind and the allegedly objective 'outer world' of physical reality?

The philosopher Rene Descartes argued that the mind must be built from some special kind of material -mind-stuff that was different from ordinary matter, indeed undetectable using ordinary matter. Mind was an invisible spiritual essence that animated otherwise unthinking matter. It was a nice idea, because it explained at a stroke why Mind is so strange, and for a long time it was the conventional view. Nevertheless, today this concept of 'Cartesian duality' has fallen out of favour. Nowadays only cosmologists and particle physicists are allowed to invent new kinds of matter when they want to explain why their theories totally fail to match observed reality. When cosmologists find that galaxies are rotating at the wrong speeds in the wrong places, they don't throw away their theories of gravitation. They invent 'cold dark matter' to fill in the missing 90 per cent of the mass of the universe. If any other scientists did that kind of thing, people would throw up their hands in horror and condemn it as 'theory saving'. But cosmologists seem to get away with it.

One reason is that this idea has many advantages. Cold dark matter is cold, dark and material.

Cold means that you can't detect it by the heat radiation that it throws off, because it doesn't.

Dark means that you can't detect it by the light that it emits, because it doesn't. Matter means that it's a perfectly ordinary material thing (not some silly invention like Descartes' immaterial mind- stuff). Having said that, of course, cold dark matter is totally invisible, and it's definitely not the same as conventional matter, which isn't cold and isn't dark ...

To their credit, the cosmologists are trying very hard to find a way to detect cold dark matter. So far, they've discovered that it does bend light, so you can 'see' lumps of cold dark matter by the effect they have on images of more distant galaxies. Cold dark matter creates mirage-like distortions in the light from distant galaxies, smearing them out into thin arcs, centred on the lump of missing mass. From those distortions, astronomers can re-create the distribution of that otherwise invisible cold dark matter. The first results are coming in now, and within a few years it will be possible to survey the universe and find out whether the missing 90 per cent of matter really is there, cold and dark as expected, or whether the whole idea is nonsense.

Descartes' similarly invisible, undetectable mind-stuff has had a very different history. At first, its existence seemed obvious: minds simply do not behave like the rest of the material world.

Then, its existence seemed obvious nonsense, because you can chop a brain into pieces, preferably after ensuring that its owner has previously departed this world, and look for its material constituents. And when you do, there's nothing unusual there. There's lots of complicated proteins, arranged in very elaborate ways, but you won't find a single atom of mindstuff[4].

We can't yet dissect a galaxy, so for now cosmologists can get away with their absurd invention of a face-saving new material. Neuroscientists, trying to explain the mind, have no such luxury. Brains are much easier to pull apart than galaxies.

Despite the change in current conventional wisdom, there remain a few diehard dualists who still believe in special mind-stuff. But today, nearly all neuroscientists believe that the secret of Mind lies in the structure of the brain, and even more importantly, in the processes that the brain carries out. As you read these words, you experience a strong sense of Self. There is a You that is doing the reading, and thinking about the words and the ideas they express. No scientist has ever dissected out the bit of the brain that contains this impression of You. Most suspect that no such bit exists: instead, you feel like You because of the overall activity of your entire brain, plus the nerve fibres that are connected to it, bringing it sensations of the outside world and allowing it to control the movement of your arms, legs and fingers. You feel like you, in fact, because you are busily being You.

Mind is a process carried out within a brain made of perfectly ordinary matter, in accordance with the rules of physics. It is, however, a very strange process. There is a kind of duality, but it is a duality of interpretation rather than of physical material. When you think a thought -about, let us say, the Fifth Elephant that slipped off the back of Great A'Tuin, orbited in an arc of a circle and crashed on to the surface of the Discworld -the same physical act of thinking that thought has two distinct meanings.

One of them is straightforward physics. In your brain, various electrons are surging to and fro in various nerve fibres. Chemical molecules are combining together, or breaking up, to make new ones. Modern sensing apparatus, such as the PET scanner[5], can reconstruct a three-dimensional image of your brain, showing which regions are active when you are thinking about that elephant. Materially, your brain is buzzing in some complicated way. Science can see how it is buzzing, but it can't (yet) extract the elephant.

That's the second interpretation. From inside, so to speak, you have no sensation of those buzzing electrons and reacting chemicals. Instead, you have a very vivid impression of a large grey creature with flappy ears and a trunk, sailing improbably through space and crashing disastrously to the ground. Mind is what it feels like to be a brain. The same physical events acquire a totally different meaning when viewed from the inside. One task of science is to try to bridge the gap between those two interpretations. The first step is to figure out which bits of the brain do what when you think a particular thought. To reconstruct, in fact, the elephant from the electrons. That's not yet possible, but every day brings it a step closer. Even when science gets there, it will probably not be able to explain why your impression of that elephant is so vivid, or why it takes exactly the form that it does.

In the study of consciousness there is a technical term for what a perception 'feels like'. It is called a quale (pronounced 'kwah-lay', not 'quail'), a figment that our minds paint on to their model of the universe in the way that an artist adds pigment to a portrait. Such qualia (plural)

paint the world in vivid colours so that we can respond more quickly to it, and, in particular, respond to signs of danger, food, possible sexual partners ... Science has no explanation of why qualia feel like they do, and it's not likely to get one. So science can explain how a mind works, but not what it is like to be one. No shame in that: after all, physicists can explain how an electron works, but not what it is like to be one. Some questions are beyond science. And, we suspect, beyond anything else: it is easy enough to claim an explanation of these metaphysical problems, but just as impossible to prove you're right. Science admits it can't handle these things, so at least it's honest.

At any rate, the science of the mind (small 'M' now because we're not talking metaphysics)

addresses how the mind works, and how it evolved, but not what it's like to be one. Even with this limitation, the science of the brain is not the whole story. There is another important dimension to the question of Mind. Not how the brain works and what it does, but how it came to be like that.

How, on Roundworld, did Mind evolve from mindless creatures?

Much of the answer lies not inside the brain, but in its interactions with the rest of the universe.

Especially other brains. Human beings are social animals, and they communicate with each other. The trick of communication made a huge, qualitative change to the evolution of the brain and its ability to house a mind. It accelerated the evolutionary process, because the transfer of ideas happens much faster than the transfer of genes.

How do we communicate? We tell stories. And that, we shall argue, is the real secret of Mind.

Which brings us back to Discworld, because on Discworld things really do work the way human minds think they do on Roundworld. Especially when it comes to stories.

Discworld runs on magic, and magic is indissolubly linked to Narrative Causality, the power of story. A spell is a story about what a person wants to happen, and magic is what turns stories into reality. On Discworld, things happen because people expect them to. The sun comes up every day because that's its job: it was set up to provide light for the people to see by, and it comes up during the day when people need it. That's what suns do; that's what they're for. And it's a proper, sensible sun, too: a smallish fire not very far away, which goes over and under the Disc, incidentally but entirely logically causing one of the elephants to lift a leg to let it pass. It's not the ridiculous, pathetic kind of sun that we have - absolutely gigantic, infernally hot, and nearly a hundred million miles away because it's too dangerous to be near. And we go round it instead of it going round us, which is crazy, especially since what every human being on the planet sees, other than the visually impaired, is the latter. It's a terrible waste of material just to make daylight

...

On Discworld, the eighth son of an eighth son must become a wizard. There's no escaping the power of story: the outcome is inevitable. Even if, as in Equal Rites, the eighth son of an eighth son is a girl. Great A'Tuin the turtle must swim though space with four elephants on its back and the entire Discworld on top of them, because that's what a world-bearing turtle has to do. The narrative structure demands it. Moreover, on Discworld everything that there is[6] exists as a thing.

To use the philosophers' language, concepts are reified: made real. Death is not just a process of cessation and decay: he is also a person, a skeleton with a cloak and a scythe, and he TALKS

LIKE THIS. On Discworld, the narrative imperative is reified into a substance, narrativium.

Narrativium is an element, like sulphur or hydrogen or uranium. Its symbol ought to be something like Na, but thanks to a bunch of ancient Italians that's already reserved for sodium

(so much for So). So it's probably Nv, or maybe Zq given what they've done to sodium. Be that as it may, narrativium is an element on Discworld, so it lives somewhere in the Disc's analogue of Dmitri Mendeleev's periodic table. Where? The Bursar of Unseen University, the only wizard insane enough to understand imaginary numbers, would doubtless tell us that there is no question: it is the umpty-umpth element.

Discworld narrativium is a substance. It takes care of narrative imperatives, and ensures they are obeyed. On Roundworld, our world, humans act as if narrativium exists here, too. We expect it not to rain tomorrow because the village fair is on, and it would be unfair (in both senses) if rain spoiled the occasion.

Or, more often, given the pessimistic ways of our country folk, we expect it to rain tomorrow because the village fair is on. Most people expect the universe to be mildly malevolent but hope it will be kindly disposed, whereas scientists expect it to be indifferent. Drought-struck farmers pray for rain, in the express hope that the universe or owner thereof will hear their words and suspend the laws of meteorology for their benefit. Some, of course, actually believe just that, and for all anyone can prove, they could be right. This is a tricky question, and a delicate one; let us just say that no reputable scientific observer has yet caught God breaking the laws of physics

(although of course He might be too clever for them) and leave it at that for the moment.

And this is where Mind takes centre stage.

The curious thing about the human belief in narrativium is that once humans evolved on the planet, their beliefs started to be true. We have, in a way, created our own narrativium. It exists in our minds, and there it is a process, not a thing. On the level of the material universe, it's just one more pattern of buzzing electrons. But on the level of what it feels like to be a mind, it operates just like narrativium. Not only that: it operates on the material world, not just the mental one: its effects are just like those of narrativium. Generally our minds control our bodies sometimes they don't, and indeed sometimes it's the other way round, especially during adolescence -and our bodies make things happen out there in the material world. Within each person there is a 'strange loop', which confuses the mental and material levels of existence.

This strange loop has a curious effect on causality. We get up in the morning and leave the house at 7.15 because we have to get to work by 9 o'clock. Scientifically, this is a very bizarre form of causality: the future is affecting the past. That doesn't normally occur in physics (except in very esoteric Quantum things, but let's not get distracted). In this case, science has an explanation.

What causes you to get up at 7.15 is not actually your future arrival at work. If in fact you fall under a bus and never make it to work, you still got up at 7.15. Instead of backwards causality, you have a mental model, in your brain, which is your best attempt to predict the day ahead. In that model, realised as buzzing electrons, you think that you ought to be at work by nine. That model, and its expectation of the future, exists now, or more accurately, a short time in the past.

It is that expectation that causes you to get up instead of lying in and having a well-deserved snooze. And the causality is entirely normal: from past to future by way of actions taking place in the present.

So that's all right then. Except that when you think of it, the causality is still very strange. A few electrons, buzzing in ways that are meaningless from the outside of the brain in which they reside, lead to a coherent action by a 70-kilogram lump of protein. Well, at that time in the morning it's not a very coherent lump of protein, but you understand what we mean. That's why we call this very creative piece of confusion a strange loop.

Those mental models are stories, simplified narratives that correspond in a rough-hewn way to aspects of the world that we consider to be important. Note that 'we': all mental models are infected with human biases. Our minds tell us stories about the world, and we base a great many of our actions on what those stories say. Here, the story is 'the person who arrived at work late and was fired from their job'. That story alone will lever us out of bed at an unearthly hour, even if we get on well with the boss and fondly imagine that the story doesn't apply to us. In other words, we make up our world according to the stories that we tell ourselves, and each other, about it.

We build minds in our children that way, too. The Western child is brought up on stories like the time Winnie the Pooh went to Rabbit's house, ate too much honey and got stuck in the entrance hole on the way out[7]. The story tells us not to be too greedy; that terrible things will happen to us if we are. Even the child knows that Winnie the Pooh is fiction, but they understand what the story is about. It doesn't lead them to avoid pigging out on honey, and it doesn't make them worry about getting stuck in the doorway when they try to leave the room after having eaten too much dinner. The story isn't about literal interpretations. It's a metaphor, and the mind is a metaphor machine.

The power of narrativium in Roundworld is immense. Things happen because of it that you would never expect from the laws of nature. For example, the laws of nature pretty much forbid an Earthbound object suddenly leaping up into space and landing on the Moon. They don't say it's impossible, but they do imply that you could wait a very long time indeed before it happened.

Despite this, there is a machine on the Moon. Several. They all used to be down here. They are there because, centuries ago, people told each other romantic tales about the Moon. She was a goddess, who looked down on us. When full, she caused werewolves to change from humans into animals. Even then, humans were quite good at doublethink; the Moon was clearly a big silver disc, but, at the same time, she was a goddess.

Slowly those tales changed. Now the Moon was another world, and by harnessing the power of swans we could fly there in a chariot. Then (Jules Verne suggested) we could get there in a hollowed-out cylinder fired by a giant gun, located in Florida. Finally, in the 1960s, we found the right kind of swan (liquid oxygen and hydrogen) and the right kind of chariot (several million tons of metal) and we flew to the Moon. In a hollowed-out cylinder, launched from Florida. It wasn't exactly a gun. Well, actually it was in a basic physical sense; the rocket was the gun and it went along for the ride, firing burnt fuel in place of a bullet.

If we'd not told ourselves stories about the Moon, there would have been no point in going there at all. An interesting view, maybe ... but we 'knew' about the view only because we had told ourselves scientific stories about images sent back by space probes. Why did we go? Because we'd been telling ourselves that we would, one day, for several hundred years. Because we'd made it inevitable and introduced it into the 'future story' of a great many people. Because it satisfied our curiosity, and because the Moon was waiting. The Moon was a story waiting to be finished ('First human lands on the Moon!'), and we went there because the story demanded it.

When Mind evolved on Earth, a kind of narrativium evolved alongside it. Unlike the Discworld variety of narrativium, which on the Disc is just as real as iron or copper or praseodymium, our variety is purely mental. It is an imperative, but the imperative has not been reified into a thing.

However, we have the sort of mind that respond to imperatives, and to many other non-things.

And so it feels to us as if our universe runs on narrativium.

There is a curious resonance here, and 'resonance' is definitely the word. Physicists tell a story about how carbon forms in the universe. In certain stars there is a particular nuclear reaction, a

'resonance' between nearby energy levels, which gives nature a stepping-stone from lighter elements to carbon. Without that resonance, so the story goes, carbon could not have formed.

Now, the laws of physics as we currently understand them involve several 'fundamental constants', such as the speed of light, Planck's constant in quantum theory, and the charge on an electron. These numbers determine the quantitative implications of the physical laws, but any choice of constants sets up a potential universe. The way that a universe behaves depends on the actual numbers that are used in its laws. As it happens, carbon is an essential constituent of all known life. All of which leads up to a clever little story called the Anthropic Principle: that it's silly for us to ask why we live in a universe whose physical constants make that nuclear resonance possible - because if we didn't, there'd be no carbon, hence no us to ask about it.

The story of the carbon resonance can be found in many science books, because it creates a powerful impression of hidden order in the universe, and it seems to explain so much. But if we look a little more closely at this story, we find that it is a beautiful illustration of the seductive power of a compelling but false narrative. When a story seems to hang together, even consciously self-critical scientists can fail to ask the question that makes it fall apart.

Here's how the story goes. Carbon is created in red giant stars by a rather delicate process of nuclear synthesis, called the triple-alpha process. This involves the fusion of three helium nuclei[8]. A helium nucleus contains two protons and two neutrons. If you fuse three helium nuclei together, you get six protons and six neutrons. That, as it happens, is a carbon nucleus.

All very well, but the odds on such a triple collision occurring inside a star are very small.

Collisions of two helium nuclei are much more common, though still relatively rare. It is extremely rare for a third helium nucleus to crash into two that are just colliding. It's like paint- balls and wizards. Every so often, a paintball will go splat! against a wizard. But you wouldn't bet a lot of money on a second paintball hitting him at the exact same moment. This means that the synthesis of carbon has to take place in a series of steps rather than all at once, and the obvious way is for two helium nuclei to fuse, and then for a third helium nucleus to fuse with the result.

The first step is easy, and the resulting nucleus has four protons and four neutrons: this is one form of the element beryllium. However, the lifetime of this particular form of beryllium is only

1016 seconds, which gives that third helium nucleus a very small target to aim at. The chance of hitting this target is incredibly small, and it turns out that the universe hasn't existed long enough for even a tiny fraction of its carbon to have been made in this way. So triple collisions are out, and carbon remains a puzzle.

Unless ... there is a loophole in the argument. And indeed there is. The fusion of beryllium with helium, leading to carbon, would occur much more rapidly, yielding a lot more carbon in a much shorter time, if the energy of carbon just happens to be close to the combined energies of beryllium and helium. This kind of near-equality of energies is called a resonance. In the 1950s Fred Hoyle insisted that carbon has to come from somewhere, and predicted that there must therefore exist a resonant state of the carbon atom. It had to have a very specific energy, which he calculated must be about 7.6 MeV[9].

Within a decade, it was discovered that there is a state with energy 7.6549 MeV. Unfortunately, it turns out that the combined energies of beryllium and helium are about 4 per cent higher than this. In nuclear physics, that's a huge error.

Oops.

Ah, but, miraculously, that apparent discrepancy is just what we want. Why? Because the additional energy imparted by the temperatures found in a red giant star is exactly what's needed to change the combined energy of beryllium and helium nuclei by that missing 4 per cent.

Wow.

It's a wonderful story, and it rightly earned Hoyle huge numbers of scientific brownie-points.

And it makes our existence look rather delicate. If the fundamental constants of the universe are changed, then so is that vital 7.6549. So it is tempting to conclude that our universe's constants are fine-tuned for carbon, making it very special indeed. An it is equally tempting to conclude that the reason for that fine-tuning is to ensure that complex life turns up. Hoyle didn't do that, but many other scientists have given into these temptations.

Sounds good: what's wrong? The physicist Victor Stenger calls this kind of argument

'cosmythology'. Another physicist, Craig Hogan has put his finger on one of the weak points.

The argument treats the temperature of the red giant and that 4 per cent discrepancy in energy levels as if they were independent. That is, it assumes that you can change the fundamental constants of physics without changing the way a red giant works. However, that's obvious nonsense. Hogan points out that 'the structure of stars includes a built-in thermostat that automatically adjusts the temperature to just the value needed to make the reaction go at the correct rate'. It's rather like being amazed that the temperature in a fire is just right to burn wood, when in fact that temperature is caused by the chemical reaction that burns the wood. This kind of failure to examine the interconnectedness of natural phenomena is a typical, and quite common, error in anthropic reasoning.

In the human world, what counts is not carbon, but narrativium. And in that context we wish to state a new kind of anthropic principle. It so happens that we live in a universe whose physical constants are just right for carbon-based brains to evolve to the point at which they create narrativium, much as a star creates carbon. And the narrativium does crazy things, like putting machines on the Moon. Indeed, if carbon did not (yet) exist, then any narrativium-based lifeform could find some way to manufacture it, by telling itself a really gripping story about the need for carbon. So causality in this universe is irredeemably weird. Physicists like to put it all down to the fundamental constants, but it's more likely an example of Murphy's law.

But that's another story.

The more we think about narrative in human affairs, the more we see that our world revolves around the power of story. We build our minds by telling stories. Newspapers select news according to its value as a story, not according to how intrinsically important it is. 'England loses cricket match to Australia' is a story (though not a very surprising one) and it goes on the front page. 'Doctors think that they may have improved the diagnosis of liver disease by 1 per cent' is not a story, even though most science works like that (and in years to come, depending on the state of your liver, you might think it's a rather more important story than a cricket match).

'Scientist claims cure for cancer' is a story, though, even if the supposed cure is nonsense. So are

'spiritualist medium claims a cure for cancer', and 'Secret code predictions hidden in the Bible', more's the pity.

As we write, there is a furore over a small group of people who are proposing to clone a human being. It's a major story, but very few newspapers are reporting the most likely result of this attempt, which will be abject failure. It took 277 failures, many rather nasty, before Dolly the Sheep was cloned, and she has now been found to have serious genetic defects, poor lamb.

Trying to clone a human may indeed be unethical, but that's not the best reason for objecting to this misguided and foolish attempt. The best reason is that it won't work, because nobody yet knows how to overcome numerous technical obstacles; moreover, if by some stroke of

(mis)fortune it did happen to work, any child produced would have serious defects. Producing such a child, now that is unethical.

Making 'carbon copies' of human beings, which is the usual basis of the newspapers' story about the ethics, is beside the point. That's not what cloning does, anyway. Dolly the Sheep was not genetically identical to her mother, though she came close. Even if she had been, she would still have been a different sheep, moulded by different experiences. For that matter, the same would be true even if she was genetically identical to her mother. For the same reason, cloning a dead child will not bring that child back to life. Much of the media discussion of the ethics of cloning, like much of the public understanding of science, is vaguely stirred through with science fiction.

In this arena, as in so many, the power of the story outweighs any questions about the real factual basis.

Human beings do not just tell stories, or just listen to them. The are more like Granny Weatherwax, who is aware of the power of stories on Discworld, and refuses to be trapped by the story's narrativium. Instead, she uses the power of story to mould events according to her own wishes. Roundworld priests, politicians, scientists, teachers and journalists have learned to use the power of story to get their messages across to the public, and to manipulate or persuade people to behave in particular ways. The 'scientific method' is a defense mechanism against that kind of manipulation. It tells you not to believe things because you want them to be true. The proper scientific response to any new discovery or theory, especially your own, is to look for ways to disprove it. That is, to try to find a different story that explains the same things.

The anthropologists got it wrong when they named our species Homo sapiens ('wise man'). In any case it's an arrogant and bigheaded thin to say, wisdom being one of our least evident features. In reality, w are Pan narrans, the storytelling chimpanzee.

At this point, the structure of The Science of Discworld 2: The Globe becomes very self- referential. You will need to bear that in mind as we proceed. The book is itself a story - no - two intertwined stories One, the odd-numbered chapters, is a Discworld fantasy. The other -the even-numbered chapters, is a story of the science of the Mind (metaphysical again). The two are closely related, designed to fit together like foot and glove[10]; the science story is presented as a series of Very Large Footnotes to the fantasy story.

So far, so good ... but it gets more complicated. When you read a Discworld story, you play a curious mental game. You react as if the story is true, as if Discworld actually exists, as if Rincewind and the Luggage are real, and Roundworld is but a fragment of a long-forgotten dream. (Please stop interrupting, Rincewind, we know it's different from your point of view. Yes, of course we're the ones that don't exist, we're bundles of rules whose consequences take place only inside a small globe on a dusty shelf in Unseen University. Yes, we do appreciate that, and will you please shut up?) Sorry about that.

People have become very good at playing this game, and we will exploit that by setting Earth and Discworld on the same narrative level, so that each illuminates the other. In the first book, The Science of Discworld, the Discworld defined what is real. That's why reality makes such good sense. Roundworld is a magical construct, designed to keep the magic out, and that's why it makes no sense at all (to wizards, at least). In this sequel Earth acquires inhabitants, the inhabitants acquire minds, and minds do strange things. They bring narrativium to a story-less universe.

A computer can do a billion sums in the blink of a keystroke and get them all right, but it couldn't pretend to be a cowardly wizard if one walked up to it and thumped it on the memory cache. In contrast, we can think ourselves inside the mind of a cowardly wizard with ease, or recognize someone else when they're acting the part of one, but we're completely lost when it comes to doing several million simple sums a second. Even though, to someone not of this universe, that might appear to be a simpler task.

That's because we run on narrativium, and computers don't.