Why do so many people dislike science fiction? The answer goes like this: You have to think of science fiction in contrast to its nearest competitor, heroic fantasy. In heroic fantasy, by and large, things are pretty stable, and then some terrible evil comes along that's going to take over the world. People have to fight it. In the end they win, of course, so the earth is restored to what it was. The status quo comes back. Science fiction's quite different. With science fiction, the world's in some sort of a state, and something awful happens. It may not be evil, it may be good or neutral, just an accident. Whatever they do in the novel, at the end the world is changed forever. That's the difference between the two genres — and it's an almighty difference! And the truth is science fiction, because we all live in a world that's changed forever. It's never going to go back to what it was in the '60s or the '70s or the '30s, or whatever. It's changed.
Brian W. Aldiss, in a Locus interview in 2000.