What is the theme in Science Fiction and Fantasy Literature by ?

Of all of the themes in Science Fiction and Fantasy, the manipulation of time has been one of the most frequently used. Most Science Fiction or Fantasy stories take place in another time, either the past or the future. In some cases, as in Wells's influential novel The Time Machine, the protagonist travels in a machine, which physically takes him either backward or forward through time, the fourth dimension. Says the time traveler, "I am afraid I cannot convey the peculiar sensations of time travelling. They are excessively unpleasant."