Robert A. Heinlein, author and critic:

Science fiction is "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" (quoted by Knight in Bishop, Nebula Awards 25, 3).

Theodore Sturgeon, author:

"'A good science-fiction story is a story about human beings, with a human problem, and a human solution, which would not have happened at all without its science content'" (as quoted in Atheling, More Issues, 12).

Brian W. Aldiss, author and critic:

"Science fiction is the search for a definition of mankind and his status in the universe which will stand in our advanced but confused state of knowledge (science), and is characteristically cast in the Gothic or post-Gothic mode" (Trillion Year Spree, 26).

Sam Moskowitz, fan and biographer:

"Science fiction is a brand of fantasy identifiable by the fact that it eases the 'willing suspension of disbelief' on the part of its readers by utilizing an atmosphere of scientific credibility for its imaginative speculations in physical science, space, time, social science, and philosophy" (Explorers of the Infinite, 11).

Judith Merril, author editor and critic:

"I use the term 'speculative fiction' here specifically to describe the mode which makes use of the traditional 'scientific method' (observation, hypothesis, experimentation) to examine some postulated approximation of reality, by introducing a given set of changes--imaginary or inventive--into the common background of 'known facts,' creating an environment in which the responses and perceptions of the characters will reveal something about the inventions, the characters, or both" (60).

Richard Hodgens, critic:

"Science fiction involves extrapolated or fictitious science or fictitious use of scientific possibilities, or it may be simply fiction that takes place in the future or introduces some radical assumption about the present or the past" (as quoted by Sobchack 19).

George Hay

"Science fiction is what you find on the shelves in the library marked science fiction."

Editors of Algol:

Science fiction is "what we mean when we point to it" (as quoted by Wertham 47).

Carlos Clarens:

"Hard to define abstractly, science fiction is instantly recognizable on the printed page."(Sobchack 19-20)

Carl D. Malmgren:

"SF does not violate notions of possibility, cause and effect, irreversibility, verifiability, and the continuity of space and time" (260)

John Clute, editor and critic:

"Fantasy. . . is a self-coherent narrative. When set in this world, it tells a story which is impossible in the world as we perceive it; when set in an otherworld, that otherworld will be impossible, though stories set there may be possible in its terms" (333)

Carl D. Malmgren:

"The worlds of [fantasy] are under no obligation to be faithful to a scientific epistemology; in such worlds, various forms of magic can govern the relations between human and natural realms, and all sorts of fantastic or impossible actants might flourish, without any scientific motivation or rationale" (260).