Science fiction is the fiction of ideas. Ideas excite me, and as soon as I get excited, the adrenaline gets going and the next thing I know I’m borrowing energy from the ideas themselves. Science fiction is any idea that occurs in the head and doesn’t exist yet, but soon will, and will change everything for everybody, and nothing will ever be the same again. As soon as you have an idea that changes some small part of the world you are writing science fiction. It is always the art of the possible, never the impossible.
Imagine if sixty years ago, at the start of my writing career, I had thought to write a story about a woman who swallowed a pill and destroyed the Catholic Church, causing the advent of women’s liberation. That story probably would have been laughed at, but it was within the realm of the possible and would have made great science fiction. If I’d lived in the late eighteen hundreds I might have written a story predicting that strange vehicles would soon move across the landscape of the United States and would kill two million people in a period of seventy years. Science fiction is not just the art of the possible, but of the obvious. Once the automobile appeared you could have predicted that it would destroy as many people as it did.
–Ray Bradbury, in The Paris Review (2010).