American science fiction author and editor Lester del Rey wrote, "Even the devoted aficionado or fan—has a hard time trying to explain what science fiction is," and the lack of a "full satisfactory definition" is because "there are no easily delineated limits to science fiction." According to Isaac Asimov, "Science fiction can be defined as that branch of literature which deals with the reaction of human beings to changes in science and technology." Robert A. Heinlein wrote that "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 scientific method."

Part of the reason that it is so difficult to pin down an agreed definition of science fiction is because there is a tendency among science fiction enthusiasts to act as their own arbiter in deciding what exactly constitutes science fiction. Damon Knight summed up the difficulty, saying "science fiction is what we point to when we say it." David Seed says it may be more useful to talk around science fiction as the intersection of other, more concrete, genres and subgenres.
Using given text, extract as many definitions of science fiction
According to Lester del Rey, "Science fiction can be defined as that branch of literature which deals with the reaction of human beings to changes in science and technology". According to Robert A. Heinlein, "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 scientific method."