Term used in this encyclopedia for the principle, formulated by H G Wells, that an sf or fantasy story should contain only a single extraordinary assumption. James Blish paraphrases it in More Issues at Hand: Critical Studies in Contemporary Science Fiction (coll 1970) as by William Atheling Jr, speaking of Wells's "hard rule ... that only a single fantastic assumption was admissible per story, and must thereafter be developed with the strictest logic of which the writer is capable." Wells's best-known statement of the "law" appears in his introduction to The Scientific Romances of H.G. Wells (omni 1933; cut vt Seven Famous Novels 1934):
... Anyone can invent human beings inside out or worlds like dumb-bells or a gravitation that repels. The thing that makes such imaginations interesting is their translation into commonplace terms and a rigid exclusion of other marvels from the story. Then it becomes human. "How would you feel and what might not happen to you," is the typical question, if for instance pigs could fly and one came rocketing over a hedge at you. How would you feel and what might not happen to you if suddenly you were changed into an ass and couldn't tell anyone about it? Or if you became invisible? But no one would think twice about the answer if hedges and houses also began to fly, or if people changed into lions, tigers, cats and dogs left and right, or if everyone would vanish anyhow. Nothing remains interesting, where anything may happen.
For the writer of fantastic stories to help the reader to play the game properly, he must help him in every possible unobtrusive way to domesticate the impossible hypothesis. He must trick him into an unwary concession to some plausible assumption and get on with his story while the illusion holds. And that is where there was a certain slight novelty in my stories when first they appeared. Hitherto, except in exploration fantasies, the fantastic element was brought in by magic. Frankenstein even, used some jiggery-pokery magic to animate his artificial monster. There was trouble about the thing's soul. But by the end of last century it had become difficult to squeeze even a momentary belief out of magic any longer. It occurred to me that instead of the usual interview with the devil or a magician, an ingenious use of scientific patter might with advantage be substituted. That was no great discovery. I simply brought the fetish stuff up to date, and made it as near actual theory as possible.
As soon as the magic trick has been done the whole business of the fantasy writer is to keep everything else human and real. Touches of prosaic detail are imperative and a rigorous adherence to the hypothesis. Any extra fantasy outside the cardinal assumption immediately gives a touch of irresponsible silliness to the invention.
There are of course many notable sf works which do not follow this precept. For example, John Wyndham's The Day of the Triffids (6 January-3 February 1951 Collier's Weekly; as "Revolt of the Triffids"; 1951; rev 1951; orig version vt Revolt of the Triffids 1952) effectively juxtaposes the hitherto only mildly dangerous Triffids with a Disaster causing near-universal blindness and giving the plant-hybrid Monsters an advantage over humanity. This double innovation is central to Wyndham's story. More generally, any sf extrapolation beyond the Near Future must for plausibility assume developments on multiple Technological and Sociological fronts, though not all need be central to the storyline; imaginative blind spots are perhaps inevitable, leading with later hindsight to Malachronism whereby – to cite a common past trope – future Spaceships are commanded and crewed by chain-smokers. [DRL]
see also: Laws.
Previous versions of this entry