I’m guessing that people will read Terry Pratchett for generations to come. Partly for the intrinsic value of the books – because they are so funny, so smart and so perceptive. And partly because, surprising as it may seem considering they are fantasy novels, there can be few better guides to contemporary thinking in the late 20th and early 21st centuries.

“He is of course writing about us,” said AS Byatt – a quote I keep coming back to because it gets to the heart of Pratchett’s achievement. He used fantasy to demonstrate all kinds of truths about mankind, including things that might otherwise have been impossible to say. To return to Night Watch and my article last week, Pratchett remarks of the people of Ankh-Morpork that they “tended to be small-minded and conservative and not very clever and were even distrustful of cleverness”. It’s harder to be so blunt about real UK citizens. Would you feel entirely comfortable suggesting as much about – for example – a place where lots of people vote Ukip (or whichever political party you like least)?

The Discworld, in short, is a good place to discuss difficult subjects. It allows for clarity and directness and also – because it is fantasy, and because the whole thing is being carried on the back of a giant turtle – sharp contrast.

A fine example of just how well the Discworld works as an idea machine comes in the Science of Discworld series co-written by Terry Pratchett, Ian Stewart and Jack Cohen. These books explore scientific ideas and tell a Discworld story in alternate chapters. The opening premise is that the wizards of Unseen University have accidentally created a new universe – one, crazily, that seems to run on the laws of physics rather than the more usual dictates of magic and narrative. Naturally, the wizards start to poke around in this universe, thus setting off the Big Bang and initiating a voyage of discovery through our own cosmic laws as they examine Earth, or “the Roundworld”, from a fresh perspective. The four books in the series have delighted and enlightened hundreds of thousands of readers (myself included) since the first one appeared in 1999.

The Science of Discworld should have been a gift to publishing. But as Jack Cohen told me last week, it took a lot of persuasion before any publisher would take it on.

“I spent two-and-a-half years going around editors,” he said. “I must have had 80 lunches and dinners. And they all said ‘don’t be stupid’. At last Ebury took it. The editor there was made to understand that if it sold less than 10,000 copies, he’d lose his job. If it sold more than 25,000 it would be a miracle. It sold more than 200,000 copies in the first year.”

Even Terry Pratchett had said: “You can’t do the science of the Discworld, because there isn’t any science on the Discworld.” But he jumped on board once a concept was worked out.

Naturally, it helped that both Cohen and Stewart had extremely distinguished scientific careers. Jack Cohen is a zoologist and embryologist whose former pupils include Nobel prize winners and a president of the Royal Society. Ian Stewart is a mathematician famous for his contributions to (and what could be more Discworldlian?) catastrophe theory. Both men already had a track record of working with science fiction writers and had written many books separately and together (including the influential The Collapse of Chaos).

Another thing that helped was that Jack Cohen had accidentally spilled a pint of beer over Terry Pratchett’s lap at a science fiction convention. The friendship was then sealed at a later convention when Cohen told Sir Terry Pratchett to “shut up”:

“It was in the Hague,” said Cohen. “Three rich science fiction authors were addressing a crowd of fans. The first one said that he sold his first story at 14 and didn’t know how much he had in the bank now – something like $2 or $3m (£1.3m or £1.9m), but that money wasn’t important. The second said he had been given $20m on his 20th birthday and that it was difficult to edit a story with real care when you have $20m in the bank. But he also went on to agree that money wasn’t important.

“Now, there were 250 fans there, most of whom had come from England to Holland, spending money, and most of whom were feeling a bit poor. And somebody threw something. The second writer dodged it.

“Then Terry came on. He started off saying, ‘Look, we’re rich science fiction authors. We don’t have to come and talk to you. We’re doing it out of the goodness of our hearts.’

“Someone threw a tomato and it got him. Terry lost it. ‘What the fuck do you think you’re doing,’ he said, and really went over the top. I stood up and said ‘shut up’. I was at the back right of the audience and all eyes turned towards me. I said, extemporising wildly: ‘Money is like air and love. If you’ve got it, it doesn’t matter. If you haven’t got it, that’s desperate.’

“Everyone stood up and clapped and Terry said: ‘Is that Jack Cohen? Then I’ll buy you a drink.’”

They got on “very well”. Soon, Terry Pratchett said he wanted to meet Cohen’s friend Ian Stewart and they all met up in a restaurant. There they started discussing a book about the science of Star Trek, which they agreed was “bloody awful”, and could have been done much better. The rest is publishing history – aside, of course, from the small matter of writing the books.

The finished books have an alternate chapter structure. One set of chapters tells a story about events on Discworld, relating to the creation and exploration of “Roundworld”. The other chapters use elements of that story to explore, well, everything from the origins of the universe onwards. The basic process was that Stewart and Cohen would write 12-14 science essays with hooks into Pratchett’s story concept. Pratchett would then work up his alternate Discworld chapters and they’d eventually put all the elements together. Some of this process involved red ink and swearing, but Cohen also tells of long pub meetings and, wonderfully, the final stages where Pratchett would introduce the “fairy dust” of jokes and substructure. “Then we sent it off to the publishers, and brilliantly, I got a third of the money.”

Naturally, Cohen said, Pratchett would be working on three other books at the same time. Pratchett, he said, was the brightest person he ever worked with, “except maybe Ian, when it comes to physics and maths”. He added: “Terry knew everything about everything. Everything you said about a painter or a novelist, he would know more. He was totally remarkable. And you could tell that from reading the books. Most readers probably only get half the jokes and references in the books, there are so many.”

That this fine mind should have been subject to the embuggerance of Alzheimer’s disease is cruel indeed. Especially so, as Cohen told me sadly, because Pratchett “had to stay around six months too long”. But even after the diagnosis he kept going, dictating to a machine, producing more fine novels, as well as working on the fourth volume of the Science of Discworld, a book which Cohen says helped turn him into “a bit of a philosopher – it deals with the big questions”.

I asked Cohen if addressing such questions had changed his outlook.

“Naturally. I change my mind all the time. I’m not a politician. Politicians seem to think it’s clever to keep the same viewpoint whatever the evidence. I don’t.”

“Is that the difference between science and politicians?” I asked.

“One difference,” said Jack, deadpan.

You can see why he got along so well with Terry Pratchett. You can see, too, why these books work so well. I’m happy to say we’re going to investigate them further in this Terry Pratchett-themed Reading group, when we run a live webchat with Ian Stewart. Please watch out for that.

One more quick bit of admin. Not everyone who successfully posted two weeks ago has claimed their copy of Night Watch. Don’t forget to get in touch.

