According to science fiction writer , "a handy short definition of almost all science fiction might read: realistic speculation about possible future events, based solidly on adequate knowledge of the real world, past and present, and on a thorough understanding of the nature and significance of the ." 's definition is "fantasy is the impossible made probable. Science fiction is the improbable made possible." wrote, "Even the devoted aficionado—or fan—has a hard time trying to explain what science fiction is", and that the reason for there not being a "full satisfactory definition" is that "there are no easily delineated limits to science fiction."
The of science fiction are often contrary to those of , but most science fiction relies on a considerable degree of , which is facilitated in the reader's mind by potential scientific explanations or solutions to various fictional elements. Science fiction elements include:
In the short run, new ideas that do not mesh well withmainstream ideas may encounter vigorous criticism, and scientistsinvestigating such ideas may have difficulty obtaining supportfor their research. Indeed, challenges to new ideas are thelegitimate business of science in building valid knowledge. Eventhe most prestigious scientists have occasionally refused toaccept new theories despite there being enough accumulatedevidence to convince others. In the long run, however, theoriesare judged by their results: When someone comes up with a new orimproved version that explains more phenomena or answers moreimportant questions than the previous version, the new oneeventually takes its place.