in Writing
Speculative fiction and the rule of one impossible thing
Many of the strongest speculative stories change one thing about the world and follow the consequences honestly. Why restraint makes the strange believable.
A common piece of advice in speculative fiction is to allow the reader one impossible thing and then play fair with everything else. The idea is often traced to H. G. Wells, who wrote about the need to domesticate an impossible hypothesis so the reader accepts it and the story can proceed.
Like most craft rules, it is better understood as a description of why certain stories work than as a law. But it describes something real.
Why one is easier to believe than many
Readers extend belief in proportion to the care they sense. When a story introduces a single strange premise and treats it seriously, the reader can hold it in mind and test the story against it. When it introduces a dozen, each arriving whenever the plot needs help, the reader stops testing and starts shrugging.
Franz Kafka's The Metamorphosis is the classic case. Gregor Samsa wakes to find himself transformed into an insect. The story never explains how. Everything that follows, his family's embarrassment, the lost income, the gradual turning of sympathy into resentment, is painfully ordinary. The impossible thing is the lens. The ordinary consequences are the story.
The consequences are the plot
The discipline the rule imposes is to think through what would follow. Say a novel imagines that people can remember one hour of the future each day. What happens to insurance, to gambling, to courtship, to grief? Who would try to exploit it, and who would be harmed? Which consequences would be obvious within a week, and which would take a generation to appear?
Stories that ask these questions carefully tend to feel inevitable. Stories that skip them tend to feel arbitrary, because the reader asks the questions anyway and notices when the book has not.
It also helps to decide what the impossible thing cannot do. Limits create stakes. A power without limits solves every problem, which leaves no story.
One premise, not one detail
The rule is sometimes misread as forbidding more than one invented element. That is not how it works in practice. A far-future novel may contain faster-than-light travel, strange aliens and unfamiliar politics. What matters is that they grow from a coherent set of assumptions, so the world feels like one place rather than a collection of convenient features.
Put another way, the budget is for unexplained leaps, not for strangeness. A richly imagined world can be very strange indeed if its parts follow from each other. What costs belief is the second or third leap that arrives unprepared, especially late in the book, especially when it rescues the characters.
When to break it
Some fiction deliberately abandons the rule. Surreal and absurdist writing may pile impossibility on impossibility, and the effect is dreamlike or comic rather than plausible. That is a legitimate choice with different aims. It works when the reader understands early that this is the kind of book they are in.
The rule is most useful for stories that want the reader to take the world seriously: to worry about it, to recognise their own world in it, or to feel that it could happen. For those stories, one impossible thing, followed honestly, is usually enough.