"[H.G. Wells said] that his method was "to trick his reader into an unwary concession to some plausible assumption and get on with his story while the illusion holds." Such prestidigitation is a characteristic ploy of science fiction: to make a nonexistent entity or impossible premise acceptable (often by scientific-sounding terms such as telepathy, extraterrestrial, cavorite, FTL speed) and then follow through with a genuinely realistic, logically coherent description of the effects and implications. Of course the accurate narrative description of the nonexistent is a basic device of all fiction. The extension to the impossible is proper to fantasy, but since we seldom know with certainty what is or is not possible, it is a legitimate element of science fiction too.