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Science fiction is difficult to define, as it includes a wide range of subgenres and themes. Author and editor Damon Knight summed up the difficulty, saying "science fiction is what we point to when we say it", a definition echoed by author Mark C. Glassy, who argues that the definition of science fiction is like the definition of pornography: you don't know what it is, but you know it when you see it. Vladimir Nabokov argued that if we were rigorous with our definitions, Shakespeare's play The Tempest would have to be termed science fiction.

As a means of understanding the world through speculation and storytelling, science fiction has antecedents back to mythology, though precursors to science fiction as literature can be seen in Lucian's True History in the 2nd century,some of the Arabian Nights tales,The Tale of the Bamboo Cutter in the 10th century and Ibn al-Nafis' Theologus Autodidactus in the 13th century.

Hard science fiction, or "hard SF", is characterized by rigorous attention to accurate detail in quantitative sciences, especially physics, astrophysics, and chemistry, or on accurately depicting worlds that more advanced technology may make possible. Many accurate predictions of the future come from the hard science fiction subgenre, but numerous inaccurate predictions have emerged as well.Some hard SF authors have distinguished themselves as working scientists, including Gregory Benford, Geoffrey A. Landis and David Brin, while mathematician authors include Rudy Rucker and Vernor Vinge. Other noteworthy hard SF authors include Isaac Asimov, Arthur C. Clarke, Hal Clement, Greg Bear, Larry Niven, Robert J. Sawyer, Stephen Baxter, Alastair Reynolds, Charles Sheffield, Ben Bova, Kim Stanley Robinson and Greg Egan.

The description "soft" science fiction may describe works based on social sciences such as psychology, economics, political science, sociology, and anthropology. Noteworthy writers in this category include Ursula K. Le Guin and Philip K. Dick. The term can describe stories focused primarily on character and emotion; SFWA Grand Master Ray Bradbury was an acknowledged master of this art. The Eastern Bloc produced a large quantity of social science fiction, including works by Polish authors Stanislaw Lem and Janusz Zajdel, as well as Soviet authors such as the Strugatsky brothers, Kir Bulychov, Yevgeny Zamyatin and Ivan Yefremov.^[50][51] Some writers blur the boundary between hard and soft science fiction.

Time travel stories have antecedents in the 18th and 19th centuries. The first major time travel novel was Mark Twain's A Connecticut Yankee in King Arthur's Court. The most famous is H. G. Wells's 1895 novel The Time Machine, which uses a vehicle that allows an operator to travel purposefully and selectively, while Twain's time traveler is struck in the head. The term "time machine", coined by Wells, is now universally used to refer to such a vehicle. Stories of this type are complicated by logical problems such as the grandfather paradox. Time travel continues to be a popular subject in modern science fiction, in print, movies, and television such as the BBCtelevision seriesDoctor Who.

Apocalyptic fiction is concerned with the end of civilization through war (On the Beach), pandemic The Last Man, astronomic impact When Worlds Collide, ecological disaster The Wind from Nowhere, or some other general disaster or with a world or civilization after such a disaster. Typical of the genre are George R. Stewart's novel Earth Abides and Pat Frank's novel Alas, Babylon. Apocalyptic fiction generally concerns the disaster itself and the direct aftermath, while post-apocalyptic can deal with anything from the near aftermath as in Cormac McCarthy's The Road to 375 years in the future as in By The Waters of Babylon to hundreds or thousands of years in the future, as in Russell Hoban's novel Riddley Walker and Walter M. Miller, Jr.'s A Canticle for Leibowitz.

The study of science fiction, or science fiction studies, is the critical assessment, interpretation, and discussion of science fiction literature, film, new media, fandom, and fan fiction. Science fiction scholars take science fiction as an object of study in order to better understand it and its relationship to science, technology, politics, and culture-at-large. Science fiction studies has a long history dating back to the turn of the 20th century, but it was not until later that science fiction studies solidified as a discipline with the publication of the academic journals Extrapolation (1959), Foundation - The International Review of Science Fiction (1972), and Science Fiction Studies (1973), and the establishment of the oldest organizations devoted to the study of science fiction, the Science Fiction Research Association and the Science Fiction Foundation, in 1970. The field has grown considerably since the 1970s with the establishment of more journals, organizations, and conferences with ties to the science fiction scholarship community, and science fiction degree-granting programs such as those offered by the University of Liverpool and Kansas University.