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.
Space travel is one of these metaphors; so is an alternative society, an
alternative biology; the future is another. The future, in fiction, is a
metaphor.

A metaphor for what?

If I could have said it non-metaphorically, I would not have written all these
words, this novel; and Genly Ai would never have sat down at my desk and used
up my ink and typewriter ribbon in informing me, and you, rather solemnly,
that the truth is a matter of the imagination.

Author: Ursula K. Le Guin