The artist deals with what cannot be said in words. The artist whose medium is fiction does this in words. The novelist says in words what cannot be said in words. Words can be used thus paradoxically because they have, along with a semiotic usage, a symbolic or metaphoric usage. All fiction is metaphor.
Science fiction is metaphor.
What sets it apart from older forms of fiction seems to be its use of new metaphors, drawn from certain great dominants of our contemporary life - science, all the sciences, and technology, and the relativistic and the historical outlook, among them. The future, in fiction, is a metaphor.