"Science fiction" is difficult to define precisely, as it includes a wide range of concepts and themes. James Blish wrote: "Wells used the term originally to cover what we would today call "hard" science fiction, in which a conscientious attempt to be faithful to already known facts (as of the date of writing) was the substrate on which the story was to be built, and if the story was also to contain a miracle, it ought at least not to contain a whole arsenal of them."
Isaac Asimov said: "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."
Tom Shippey compared George Orwell's Coming Up for Air (1939) with Frederick Pohl and C. M. Kornbluth's The Space Merchants (1952), and concluded that the basic building block and distinguishing feature of a science fiction novel is the presence of the novum, a term Darko Suvin adapted from Ernst Bloch and defined as "a discrete piece of information recognizable as not-true, but also as not-unlike-true, not-flatly- (and in the current state of knowledge) impossible."
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." Author and editor Damon Knight summed up the difficulty, saying "science fiction is what we point to when we say it." Mark C. Glassey described science fiction as U.S. Supreme Court Justice Potter Stewart described pornography: "I know it when I see it."