Science fiction is a genre of fiction dealing with imaginary but more or less plausible (or at least non-supernatural) content such as future settings, futuristic science and technology, space travel, aliens, and paranormal abilities. Exploring the consequences of scientific innovations is one purpose of science fiction, making it a "literature of ideas.” Science fiction is largely based on writing rationally about alternative possible worlds or futures. It is similar to, but differs from fantasy in that, within the context of the story, its imaginary elements are largely possible within scientifically established or scientifically postulated laws of nature (though some elements in a story might still be pure imaginative speculation). The settings for science fiction are often contrary to known reality, but most science fiction relies on a considerable degree of suspension of disbelief, which is facilitated in the reader's mind by potential scientific explanations or solutions to various fictional elements. Science fiction elements include: A time setting in the future, in alternative timelines, or in a historical past that contradicts known facts of history or the archaeological record. A spatial setting or scenes in outer space (e.g., spaceflight), on other worlds, or on subterranean earth. Characters that include aliens, mutants, androids, or humanoid robots. Technology that is futuristic (e.g., ray guns, teleportation machines, humanoid computers). Scientific principles that are new or that contradict known laws of nature, for example time travel, wormholes, or faster-than-light travel. New and different political or social systems (e.g. dystopia, post-scarcity, or a post-apocalyptic situation where organized society has collapsed). Paranormal abilities such as mind control, telepathy, telekinesis, and teleportation. 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 by stating that "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. According to science fiction writer 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." Rod Serling's definition is "fantasy is the impossible made probable. Science Fiction is the improbable made possible." Lester del Rey 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."