We keep coming back to this. Consistency is just as important in fantasy as it is in science fiction. When you create a world, you create an illusion in the reader's mind. You're like a wizard casting a spell. But if the sun sets in the west on one page and in the east on another, and there's no good reason for it, you can ruin the illusion in a heartbeat. Ursula K. LeGuin refers to that as taking the reader "from Elfland to Poughkeepsie" — meaning that you jar the reader out of the story and back into the real world.

Be consistent, and maintain the illusion!

Plausibility

In science fiction, there's a particular challenge, and that's the need for scientific plausibility. In other words, it has to make sense scientifically; it has to be believable.

That's not quite the same thing as scientific accuracy. Science fiction is often based on taking known scientific principles and stretching them a little, bending here and twisting there. A good example is faster than light space travel (FTL). Current scientific thinking says that there is no known way to travel faster than light.

I emphasize known. From a fictional point of view, you can violate that law, but only by proposing some future discovery that permits FTL, such as space warps or wormholes or hyperdrive.

It's another way of maintaining the illusion. If you bend science, do it knowingly. Have in mind a good explanation for it. That doesn't mean you necessarily have to spell it out for the reader — we don't need an explanation in every book about how a starship's hyperdrive works, unless it's a really interesting explanation. But if it's clear in your mind, there's a good chance it will feel real in the reader's mind.

[See also Part 9: Research and Believability]