The word 'lie' has negative overtones, deservedly so, but what we're talking about here can be constructive as well as destructive, and often is. For the purposes of the present discussion, a lie is anything contrary to the truth, but it's not at all clear what 'the truth' is, or even whether there is only one of it, as the word 'the' would seem to indicate. When two people have a row, it is generally impossible for either of them, or anyone else, to figure out exactly what really happened. Our thoughts are tainted by perceptions. This is unavoidable, because what we think of as being 'real' is what our minds make of what comes from the sense organs: fudged, tuned, and mangled by a succession of interpretations by different bits of brain, plus some wallpaper additions. We never know what is really out there around us. All we know is what our minds construct from what our eyes, ears and fingers report.

Not to put too fine a point on it, those perceptions are lies. The vivid universe of colour that our brain derives from the light that falls on our retinas does not really exist. The redness of a rose is derived from its physical features, but 'being red' is not a physical feature as such. 'Emitting light of a certain wavelength' gets closer to being a physical feature. However, the vivid redness that we 'see' does not correspond to a specific wavelength. Our brains correct the colours of visual images for shadows, light reflected on to parts of the image by other parts of a different colour, and so on. Our sensation of redness is a decoration added to the perception by our brain: a quale.

So what we 'see' is not an accurate perception of what is there, but a mental transform of a sensory perception of what is there.

To a bee, that same uniformly red rose may look very different, with obvious markings. The bee

'sees' in ultraviolet, a wavelength outside our range of perceptions. The rose emits a whole distribution of wavelengths of light; we see a small part of that, and call it reality. The bee sees a different part and responds to it in its own beelike way, using the markings to land on the flower and collect nectar, or to dismiss it from consideration and fly on to the next possibility. Neither the bee's perception, nor ours, is the reality.

In Chapter 24 we explained that our minds select what they perceive in more ways than just passively ignoring signals that our senses can't pick up. We fine-tune our senses to see what we want them to see, hear what we want them to hear. There are more nerve connections going from the brain to the ear than there are from the ear to the brain. Those connections adapt the ear's ability to perceive certain sounds, maybe by making it more sensitive to sounds that could represent danger and less sensitive to sounds that don't really matter much. People who are not exposed to certain sounds as children, when their ears and brains are being tuned to pick up language, cannot distinguish them as adults. To the Japanese, the two phonemes 'l' and 'r' sound identical.

The lies that our senses tell us are not malicious. They are partial truths rather than untruths, and the universe is so complicated, and our minds are so simple in comparison, that the best we can ever hope for is half-truths. Even the most esoteric 'fundamental' physics is at best a half-truth.

Indeed, the more 'fundamental' it becomes, the less true it gets. It is therefore no surprise that the most effective method we have yet devised for passing extelligence on to our children is a systematic series of lies. It is called 'education'.

We can hear the hackles rising even as we write, as quantum signals echo back down the timelines from future readers in the teaching profession turning to this page. But before hurling the book across the room or sending an offended e-mail to the publisher, ask yourself just how much of what you tell children is true. Not worthy, not defensible: true. At once you'll find yourself on the defensive: 'Ah, yes, but of course children can't understand all of the complexities of the real world. The teacher's job is to simplify everything as an aid to understanding ...' Quite so.

Those simplifications are lies, within the meaning we are currently attaching to that word. But they are helpful lies, constructive lies, lies that even when they are really very wrong still open the door to a better understanding next time round. Consider, for example, the sentence 'A

hospital is a place where people are sent so that the doctors can make them better'. Well, no sensitive adult would wish to tell a child that sometimes people go into hospital alive and come out dead. Or that often it's not possible to make them better. For a start, the child may have to go into hospital at some stage, and too big a dose of truth early on might make it difficult for the parents to persuade them to do so without making a fuss. Nonetheless, no adult would consider that sentence to be an accurate statement of what hospitals are really about. It is, at best, an ideal to which hospitals aspire. And when we justify our description on the grounds that the truth would upset the child, we are admitting that the sentence is a lie, and asserting that social conventions and human comfort are more important than giving an accurate description of what the world is about.

They often are, of course. A lot depends on context and intention. In Chapter 4 of The Science of Discworld we called these helpful untruths and half-truths 'lies-to-children'. They must be distinguished from the much less benevolent 'lies-to-adults', another word for which is 'politics'.

Lies-to-adults are constructed with the express purpose of concealing intentions; their aim is to mislead. Some newspapers tell lies-to-adults; others do their best to tell truths-to-adults, although they always end up by telling adult versions of lies-to-children.

In the twenty-fifth Discworld novel The Truth, journalism comes to the Disc, in the form of William de Worde. His career begins with a monthly newsletter sent to various Discworld notables, usually for five dollars each month, but in the case of one foreigner for half a cartload of figs twice per year. He writes one letter, and pays Mr Cripslock the engraver in the Street of Cunning Artificers to turn it into a woodcut, from which he prints five copies. From these small beginnings emerges Ankh-Morpork's first newspaper, when de Worde's ability to sniff out a story is allied to the dwarves' discovery of movable type. It is rumoured that the dwarves have found a way to turn lead into gold -and since the type is made of lead, in a way they have.

The main journalistic content of the novel is a circulation battle between de Worde's Ankh- Morpork Times, with its banner 'THE TRUTH SHALL MAKE YOU FREE', and the Ankh- Morpork Inquirer (THE NEWS YOU ONLY HEAR ABOUT). The Times is an upmarket broadsheet, running stories with headlines like 'Patrician Attacks Clerk With Knife (He had the knife, not the clerk)', and checking its facts before publishing them. The Inquirer is a tabloid, whose headlines are more of the 'ELVES STOLE MY HUSBAND' kind, and it saves money by making all the stories up. As a result, it can undercut its upmarket competitor when it comes to price, and the stories are much more interesting. Truth eventually prevails over cheap nonsense, however, and de Worde learns from his editor Sacharissa a fundamental principle of journalism:

'Look at it like this,' said Sacharissa, starting a fresh page. 'Some people are heroes. And some people jot down notes.'

'Yes, but that's not very—'

Sacharissa glanced up and flashed him a smile. 'Sometimes they're the same person,' she said.

This time it was William who looked down, modestly.

'You think that's really true?' he said.

She shrugged. 'Really true? Who knows? This is a newspaper, isn't it? It just has to be true until tomorrow.'