“Introduction”, Damon Knight, One Hundred Years of Science Fiction, book two.

I have said elsewhere that science fiction and prophecy are two different things. Science fiction deals with what may be, not what will be. It is true that we can now afford to smile at those who once dismissed SF as ‘Buck Rogers stuff’, while our astronauts orbit the earth, our space probes photograph the moon, Mars, and Venus; while computers take over banking, commerce and industry, and electron microscopes unlock the secrets of the genetic code. But to value science fiction merely because of its predictive quality is as absurd as to reject it, as some unimaginative readers do, because it is ‘not true’. Science fiction stories allow us to enter worlds that exist nowhere else. We prize them for that, not for the irrelevant coincidence that may later make them seem prophetic. The War of the Worlds is no less a great story because the Martians have not landed; neither is Out of the Silent Planet diminished if there are no sorns.

(Click for full graphic of the opposite point of view.)