THINGS THAT AREN'T
LIGHT HAS A SPEED, SO WHY NOT DARK?
It's a reasonable question. Let's see where it leads. In the 1960s a biological supply company advertised a device for scientists who used microscopes. In order to see things under a microscope, it's often a good idea to make a very thin slice of whatever it is you're going to look at. Then you put the slice on a glass slide, stick it under the microscope lens, and peer in at the other end to see what it looks like. How do you make the slice? Not like slicing bread. The thing you want to cut, let's assume it's a piece of liver for the sake of argument, is too floppy to be sliced on its own.
Come to think of it, so is a lot of bread.
You have to hold the liver firmly while you're cutting it, so you embed it in a block of wax. Then you use a gadget called a microtome, something like a miniature bacon-slicer, to cut off a series of very thin slices. You drop them on the surface of warm water, stick some on to a microscope slide, dissolve away the wax, and prepare the slide for viewing. Simple...
But the device that the company was selling wasn't a microtome: it was something to keep the wax block cool while the microtome was slicing it, so that the heat generated by the friction would not make the wax difficult to slice and damage delicate details of the specimen.
Their solution to this problem was a large concave (dish-shaped) mirror. You were supposed to build a little pile of ice cubes and 'focus the cold' on to your specimen.
Perhaps you don't see anything remarkable here. In that case you probably speak of the 'spread of ignorance', and draw the curtains in the evening to 'keep the cold out', and the darkness.
In Discworld, such things make sense. Lots of things are real in Discworld while being mere abstractions in ours. Death, for example. And Dark. On Discworld you can worry about the speed of Dark, and how it can get out of the way of the light that is ploughing into it at 600 mph. In our world such a concept is called a 'privative', an absence of something. And in our world, privatives don't have their own existence. Knowledge does exist, but ignorance doesn't; heat and light exist, but cold and darkness don't. Not as things.
We can see the Archchancellor looking puzzled, and we realize that here is something that runs quite deep in the human psyche. Yes, you can freeze to death, and 'cold' is a good word for describing the absence of heat. Without privatives, we would end up talking like the pod people from the Planet Zog. But we run into trouble, though, if we forget that we're using them as an easy shorthand.
In our world there are plenty of borderline cases. Is 'drunk' or 'sober' the privative? In Discworld you can get 'knurd', which is as far on the other side of sober as drunk is on the inebriated side, but on planet Earth there's no such thing. By and large, we think we know which member of such a pairing has an existence, and which is merely an absence. (We vote for 'sober' as the privative. It is the absence of drink, and, usually, the normal state of a person. In fact that normal state is only called sobriety when the subject of drink is at hand. There's nothing strange about this. 'Cold' is the normal state of the universe, after all, even though as a thing it does not exist. Er ... we're not going to get past you on this one, are we, Archchancellor?)
Thinking is required if our language isn't to fool us. However, as 'focusing the cold' shows, we sometimes don't stop to think.
We've done it before. At the start of the book, we mentioned phlogiston, considered by early chemists to be the substance that made things burn. It must do: you could see the phlogiston coming out as flames, for goodness' sake. Gradually, however, clues that supported the opposite view accumulated. Things weigh more after they've burned than they did before, for instance, so phlogiston seemed to have negative weight. You may think this is wrong, incidentally; surely the ash left by a burnt log weighs a lot less than the log, otherwise nobody would bother having bonfires? But a lot of that log goes up in smoke, and the smoke weighs quite a bit; it rises not because it's lighter than air but because it's hot. And even if it were lighter than air, air has weight, too. And as well as the smoke, there's steam, and all sorts of other junk. If you burn a lump of wood, and collect all the liquids, gases, and solids that result, the final total weighs more than the wood.
Where does the extra weight come from? Well, if you take the trouble to weigh the air that surrounds the burning wood, you'll find that it ends up lighter than it was. (It's not so easy to do both of these weighings while keeping track of what came from where -think about it. But the chemists found ways to achieve this.) So it looks as if something gets taken out of the air, and once you're realized that's what's going on, it's not hard to find out what it is. Of course, it's oxygen. Burnt wood gains oxygen, it doesn't lose phlogiston.
This all makes far more sense, and it also explains why phlogiston wasn't such a silly idea. Negative oxygen, oxygen that ought to be present but isn't, behaves just as nicely as positive oxygen in all the balancing equations that chemists used to check the validity of their theories. So much phlogiston moving from A to B has exactly the same effect on observations as the same amount of oxygen moving from B to A. So phlogiston behaved just like a real thing, with that embarrassing exception that when your measurements became accurate enough to detect the tiny amounts involved, phlogiston weighed less than nothing. Phlogiston was a privative.
A difficult but stubborn feature of human thinking is involved in all this: it's known as 'reifying': making real. Imagining that because we have a word for something, then there must exist a 'thing' that corresponds to the word. What about 'bravery' and 'cowardice'? Or 'tunnel'? Indeed, what about 'hole'?
Many scientific concepts refer to things that are not real in the everyday sense that they correspond to objects. For instance, 'gravity' sounds like an explanation of planetary motion, and you vaguely wonder what it would look like if you found some, but actually it is only a word for an inverse square law attractive relationship. Or more recently, thanks to Einstein, for a tendency of objects not to move in straight lines, which we can reify as 'curved space'.
For that matter, what about 'space'? Is that a thing, or an absence?
'Debt' and 'overdraft' are very familiar privatives, and the thinking problems they cause are quite difficult. After all, your overdraft pays your bank manager's salary, doesn't it? So how can it fail to be real? Today's derivatives market buys and sells debts and promises as if they were real, and it reifies them as words and numbers on pieces of paper, or digits in a computer's memory. The more you think about it, the more amazing the everyday world of human beings becomes: most of it doesn't actually exist at all.
Some years ago, at a science-fiction convention held in The Hague, four writers who made lots of money from their books sat in front of an audience of mostly impecunious fans to explain how they'd made huge income from their books (as if any of them really knew). Each of them said that 'money isn't important', and the fans became quite rude at this perfectly accurate statement. It was necessary to point out that money is like air or love, unimportant if you've got enough of it, but desperately important if you haven't, Dickens recognized this: in David Copperfield Mr Micawber remarks 'Annual income twenty pounds, annual expenditure nineteen nineteen six, result happiness. Annual income twenty pounds, annual expenditure twenty pounds ought and six, result misery.'