Forty years ago and more [since circa 1926], science-fiction writers seemed to be saying "People will reach the moon," and today this looks like dazzling prophecy. What they really were saying, however, was something slightly different and much more significant: "People can reach the moon if they want to badly enough." ... — and here, finally, we come to something of which I think science-fiction writers can be justly proud.
What science fiction has been doing ... is to shake up people's thinking, make them skeptical of dogma, get them used to the idea of change, let them dare to want new things. Nobody will ever know for sure how much effect these stories have had, but it is almost impossible to believe they have had none.
"What is Science Fiction, Anyway?"
In Search of Wonder