In fantasy fiction, that things happen that could not happen in the real world. That’s the essence of fantasy, and it’s obvious. There could be dragons, ghosts, zombies, unicorns, you name it.
What’s perhaps less obvious is that in fantasy, most of the things that happen are exactly what one would expect to happen in the real world. Gravity works as expected. Digestion works as expected. Conversations and facial expressions work as expected. Money, clothing, weather … everything is normal, except the parts of the story that aren’t.
In constructing a fantasy world, the writer begins with a counter-factual premise. That premise leads to certain observable phenomena (hauntings, the drinking of blood, the ability to fly through the air on a winged horse), which the writer will work out, we hope, with enough care to make them internally consistent and therefore believable. But anything in the fantasy world that doesn’t derive from the premise remains normal.
A classic example is to be found in Naomi Novik’s novel His Majesty’s Dragon. The setting is the early 19th century on Earth, and everything is exactly as we know it from the historical record — except, there are dragons. The rules for what dragons can do and how people interact with dragons are entirely fantastical, but nothing else is even remotely off-kilter.
The one apparent exception — and it’s more apparent than real — would be a dream-world story like Alice’s Adventures in Wonderland. In this story, animals can talk, people can change size, and a flamingo can be used (awkwardly) as a croquet mallet. But the premise underlying the story is simple. The premise is, implausible things can happen. But except for the implausible things, everything is normal. If it weren’t, the story would make no sense. In particular, Alice is a very ordinary little girl. She doesn’t suddenly start speaking Russian or chew on the Mad Hatter’s face.
I started ruminating about what a fantasy premise does and does not do following a conversation on Facebook. Somebody had posted something on a fantasy fiction page about Stephen Donaldson’s series on Thomas Covenant. I replied that I lost interest in the story midway through the first book (Lord Foul’s Bane). Thomas was camped out in the forest and having trouble sleeping, so he was gazing up at the night sky. Donaldson told the reader that a full moon rose at midnight and set again long before dawn.
This is flatly impossible. The celestial mechanics just don’t work! The moon is full when it’s at the point in its orbit opposite the sun. A full moon can rise only at sunset; it can’t rise in the middle of the night. Also, the moon crosses the night sky at approximately the same rate as the stars. If we assume the night is 12 hours long, the moon will set 12 hours (actually a tiny bit more) after it rose, not four or five hours. If the moon were moving that quickly across the sky, it would have to be much closer to the Earth, because gravity is the same in a fantasy world. If the moon were that much closer, it would also have to be a lot smaller. Now, there’s no reason to assume a moon couldn’t be smaller and closer to Earth, that’s not the problem. The problem is, if it were closer to the Earth it would be in the Earth’s shadow for much of the night. It would be eclipsed. Not full to start with, and then eclipsed.
Evidently Donaldson was ignorant of celestial mechanics. So I put the book down and never touched it again.
Predictably, somebody in the Facebook thread replied that the story is fantasy, and the moon might do something different in Donaldson’s world. But that argument is fallacious, because the movement of the moon in its orbit has nothing to do with the story premise. As a writer, you don’t get to change just anything you happen to feel like and then say, “Oh, but it’s fantasy!”
You can do absolutely anything in a fantasy story — except that you can’t violate your own premise. The impossible things must arise from the premise. Terry Pratchett’s Discworld books are a good example. Discworld is packed with impossible fantasy stuff. The world itself is flat, and rests on the backs of four unimaginably large elephants, who are standing on the back of a giant turtle who swims through space. There are vampires, zombies, wizards, witches, dwarfs, trolls, a werewolf, and a talking dog named Gaspode. There’s a very skinny guy with a permanent toothy grin, who carries a scythe, shows up when you die, and ALWAYS SPEAKS IN CAPITAL LETTERS. He puts in an appearance in just about every Discworld book, and he has a granddaughter named Susan. (She’s adopted.) He rides a white horse called Binky.
Pratchett’s premise is something like, “This is a world in which fantasy elements from a bunch of different human cultures are all real and are all packed in together.”
Within that wild and wacky context, however, a lot of things are very normal. Sergeant Angua is a werewolf, but when she transforms back into her human form she’s naked, because wolves don’t wear clothing and have no way to carry any. The trolls and the dwarfs don’t like one another, and while their racial characteristics are fantastical, their feuding is very much like what we can imagine happening between rival gangs in any large city on Earth; it’s exaggerated, but the emotional attitudes are realistic. Sam Vimes loves his son, and his expressions of that fact are completely normal. He doesn’t love his son one day and not recognize him the next day; that would violate the fantasy premise.
With apologies to Robert Heinlein, the moon is a harsh mistress. You can have dragons, but you have to get the moon right.