Is HDM a work of Science Fiction? No, you say, it contains no spaceships or time machines, no robots or ion blasters.

Yes, that’s right. Neither Anakin Skywalker nor Mr Spock feature in Philip Pullman’s novel. And yet, the story contains Science Fictional elements and they are worth taking a look at.

What does the expression "Science Fiction" mean, anyway? Does it automatically imply Spandex jumpsuits and hyperdrives? Well, no. There was a move among SF intellectuals in the 1960s/70s to change the genre’s name to "Speculative Fiction". That move failed, but the nature of what we call SF was already changing to match the new name and it has continued to do so.

Not that the phasers and Dilithium crystals have disappeared, I’m glad to say. But a more thoughtful and more literary style manifested itself in the field around that time and it’s stuck since. SF (whatever that stands for) could be renamed "WIF" – for "What-If Fiction".

Before I expand on that statement; a word about genre fiction. SF is a genre fiction. So are detective novels, historical novels. Mills and Boon romances and others. The distinguishing feature of a genre is that it has a set of underlying rules which the writer is expected to follow. For example, a Mills and Boon romance will go; girl-meets-boy, girl-loses-boy, girl-gets-boy-back, clinch-at-the-end. A classical detective story will introduce the perpetrator at the start, there will be red herrings, and the reader will have enough evidence to finger the perp himself before the denouement in the final chapter.

These rules – correctly followed – are no more restrictive than the everyday rules of grammar or spelling. You can break them if you want, but you’d better have a very good reason to do so. It’s notable that non-genre writers often make a bit of a hash of it when they try to write within a genre. For example, Doris Lessing’s Canopus in Argos books are not, in my opinion, successful either as SF or as mainstream literary fiction. It’s also notable that SF and mystery writers find it easy to write in each other’s genres. Isaac Asimov, for example, wrote good SF mysteries (The Caves of Steel, for example). Why should this be so?

I would say that it is down to a question of consistency. Non-genre writers fall down when writing in a genre because they are inconsistent in applying the rules of that genre. For example, a locked-room mystery is solved because someone is revealed (on page 230) to be an inventor who has created a teleportation device. Or the robots hate humans for no good reason (the writer read Frankenstein once). And so on – I won’t name names.

Let’s go back to What-If. When a SF writer wants to write a new story, he or she generally starts with a What-If? For example:

• What if I could make a machine that would let me travel into the future?
• What if the Japanese had won World War 2?
• What if personal teleportation became cheap, reliable and safe?

Then the writer asks the second and much more important question:

• What is the result of my What-If on the characters in my story and the world they inhabit?

And the result is The Time Machine, by HG Wells, or The Man in the High Castle, by Philip K Dick, or All the Bridges Rusting, by Larry Niven.

You’ll notice, by the way, that there is no inherent science or technology component in our second What-If. That’s one reason why the Science nearly got taken out of Science Fiction.

Philip Pullman asks several What-Ifs in HDM:

• What if a device could be made that would allow people to travel between the different worlds of probability?
• What if, in one of those worlds, people were accompanied by their souls or consciences in daemon-form?
• What if angels are real, and constitute the dark matter of the universe?

Let’s look at how well he manages that important second question, starting with Lyra’s world. The Subtle Knife is at the core of the nature of the worlds. Pullman supposes that the very act of making the Knife, as well as the consequences of its use, upsets the natural balance of the worlds which comprise his multiverse. The Reformation goes astray in Lyra’s world, the Cittagazzeans get lazy in theirs, and spirituality takes second place to the Enlightenment in our world.

Lyra’s world is recognisably our world at the turn of the 19th and 20th centuries, with a few twists. The first thing the SF reader notices is that technologically it’s rather confused. There are hydrogen-filled airships, atomic power, "gas-engines" and a Chthonic Railway that the upper classes disdain to use, but also naphtha lighting. It’s a characteristic of non-SF writers when they do this sort of world-building that they tend to slip up when it comes to managing what you might call the hierarchy of technology. We are given to understand that the Magisterium closely controls all scientific ("practical theology") investigation and that heretical lines of enquiry are brutally suppressed. Such a regime tends to stifle scientific progress. Think of Galileo in our world, forced to disclaim his discoveries about the structure of the Solar System. And yet, there is nuclear power ("atomcraft") on a commercial level (it’s one of Jordan College’s interests). You could argue that the Church’s advanced research into fundamental particles has led to the early discovery of atomic energy, but this ignores all the other things you need in order to run a nuclear power station. You need a well-developed chemical industry, for a start. Some knowledge of electronics would be handy, not to mention precision engineering. In fact you need an advanced industrial infrastructure, and there are few signs of this. Why is there no radio? Moving photograms should be commonplace and, they, like radio, should be under the control of the Magisterium, just as they were in Nazi Germany and the old Communist countries, who loved the idea of a direct propaganda link down from their rulers to their subjects.

Lyra’s Brytain may be a little shaky conceptually, but the world of Cittagazze positively totters. We are expected to believe that in this world, where adults are dropping left, right and centre, victims of the Spectres, there are factories where they make clothing for sale (Lyra’s skirt), ready supplies of cooking gas, and electricity to work the lights and, to cap it all, refrigerators where they keep their stocks of canned fizzy drinks! I can only suppose that there is a window that leads directly from Cittagazze to a Coca-Cola plant. And perhaps they’re leeching electrical power from the generating station in our Didcot, Oxfordshire.

It’s a shame – Cittagazze is a beautifully evoked world in so many ways, but it’s shoddily built. One push and it falls apart.

What about the great McGuffin – the Subtle Knife itself? Paradoxically, there’s no need to explain or justify its capabilities. It is what it is – a magical device that does what it does. You could try to explain how it works in scientific terms, but that wouldn’t make it any more credible. If anything, it would have the opposite effect. Pseudo-scientific variants on the Knife theme (like energised Buckythread or the Trans-Dimensional Resonator) are no better than the original.

The second What-If, the presence of external daemons in Lyra’s world, is pretty well handled by Pullman. He has a multitude of examples, from the talking animals of fairy-tales through to Anne MacCaffrey’s dragons, to draw from, but his variant is recognisably his own, and original. The only quibble I have is that I would have expected Pullman to have sat down before he started on Northern Lights and write down everything he knew about daemons. I don’t think he did, and in evidence I would cite the interview where he said that he knew nothing of daemon gynaecology. Well heck, Philip, why on earth not?

And lastly, the angels. We’re in the realm of metaphysics here, but that doesn’t mean that the requirements of consistency don’t matter. Again, on the whole, Pullman does well. The angels’ desire to be more material leads to an apparent inconsistency in their physical capabilities, but this inconsistency is more apparent than real, I think.

There are some other points, but in the overall scheme of things they tend to pass by without causing too many problems. Just for an example, if Asriel’s bridge is so energetically disruptive that it causes major climate changes in the northern hemisphere, how come it is possible for unprotected humans to live in its vicinity? The Tunguska asteroid didn’t particularly affect our climate, but I wouldn’t have wanted to be close by when it struck!

I guess that only someone like me, who likes to know how things work, would have picked up on these points. HDM is a metaphysical speculation, not a work of hard Science Fiction, and to accuse it of a failure to stick closely to the rules of a genre to which it is only allied is being unfair. But Science Fiction and genre fiction generally, have lessons in terms of background solidity and consistency of invention that mainstream and literary fiction – and fantasy too – should try to learn.