My bookshelves are bastions of unreality. Narnia and Middle-Earth, Dune and the Culture. Morgan and Mieville. Gaiman and Pullman. Joe Hill and H.P. Lovecraft. The direction of my reading has been second to the right and straight on ’til morning since I was young enough to want to be Wendy.

Most bookshops stack the stuff I love in a corner labelled Fantasy/Sci Fi/Horror, and they do indeed sit very comfortably on a continuum of related reading experiences. As a reader, I never thought too much about the underlying structure of the fantastical. As a writer, I have to. And I’ve discovered that the distinctions which are often subtle for the reader can be quite profound for the writer.

All three genres posit a reality that is different than the humdrum, everyday “real world” that we all inhabit; the writer has to create that reality and draw the reader into it. This is worldbuilding, and while it’s a necessary element of almost every story, its demands on the imagination are arguably greater for horror, fantasy and science fiction than for other genres. But there are some key differences between these broad categories of the unreal. I find them in the measure of internal coherence required of the fictional world; the degree of continuity between it and the “real” world; and the amount of explanation that needs to be provided to the reader.

In horror, the reader is given little or no information about the hidden mechanics of the storyworld; it often appears to be the same as the “real” world (and therefore to require no explanation), until weird things start to happen. Then the inexplicability of events, and their disconnection from a rational, coherent framework wherein they make sense in relation to other events is what drives the sense of apprehension and terror. (A caveat: this applies more to modern horror writing. Classic novels such as Frankenstein and Dracula were written following what we would now think of as a science fiction or fantasy approach to worldbuilding.)

In fantasy, the reader is given a greater degree of explanation for how the world of the story works, which is necessary as it is usually immediately obvious that it is not the “real” world. These explanations are often elaborate and detailed, but they only need to be internally coherent – in other words they only need to make sense within the covers of the book, within the world of the story. The laws and logic of the fantasy world can be completely disconnected from the “real” world, as long as the story obeys the special rules of the fantasy world.

In science fiction lots of explanation is required, and it needs to be both internally coherent and to have some continuity with the “real” world. The physical reality of the science fiction story needs to follow the same basic rules as the “real” world, or at any rate to provide a rational explanation for any discrepancies. Science fiction need not always be set in the future; but wherever and whenever the story occurs, and however profoundly different the world it inhabits, the reader needs a plausible connection between the “here” of the real world and the “there” of the science fiction world. A fantasy world does not require the same degree of plausibility.

It seems to me that this sequence represents an ascending order of difficulty for the writer as worldbuilder. In horror the storyworld does not need to make rational sense; in fantasy it needs to make sense internally, but not externally; in science fiction it needs to be plausible both internally and externally. (I’m not, by the way, suggesting that it’s easier to write horror than science fiction – far from it. Creating the suspension of disbelief necessary to make you scared of an implausible monster is a tough trick.)

Having managed to unpick the nature of the writers’ challenges and readers’ complicity in constructing these imaginary settings, we inevitably run up against the stuff that just doesn’t seem to fit. Novels like The Time Traveller’s Wife or Never Let Me Go create a problem for genre cubbyholers. The scenarios they posit should be classic science fiction Big Ideas – but the authors make no real attempt to explain the how or why of the situations the characters find themselves in. Their worlds could be ours, but for the disconnects – an unrecognisable history that just is, seemingly impossible stuff that just happens. And excellent and acclaimed though these books are, there remains a sneaking sense of unease amongst both the SF geeks who want an explanation, dammit, and aren’t entirely inclined to trust an author who doesn’t give them one; and the snobbish literati who can’t quite put to bed the suspicion that they’ve been conned into reading something that smacks – gasp, shudder – of sci fi. Horrors.

Which, actually, is pretty close to the mark. I’m not a genre pedant; I’m happy to simply read a good book. But if I had to shelve these two in my fantasy bookshop of the fantastical, under Horror they would go.