From Sam Weller’s interview of Ray Bradbury in “The Art of Fiction No. 203” (The Paris Review: Spring 2010, No. 192):
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.
I often use the metaphor of Perseus and the head of Medusa when I speak of science fiction. Instead of looking into the face of truth, you look over your shoulder into the bronze surface of a reflecting shield. Then you reach back with your sword and cut off the head of Medusa. Science fiction pretends to look into the future but it’s really looking at a reflection of what is already in front of us. So you have a ricochet vision, a ricochet that enables you to have fun with it, instead of being self-conscious and superintellectual.