In The Paris Review, Ray Bradbury expounds:

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.

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[M]ainstream [fiction] hasn’t been paying attention to all the changes in our culture during the last fifty years. The major ideas of our time—developments in medicine, the importance of space exploration to advance our species—have been neglected. The critics are generally wrong, or they’re fifteen, twenty years late. It’s a great shame. They miss out on a lot. Why the fiction of ideas should be so neglected is beyond me. I can’t explain it, except in terms of intellectual snobbery.

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Take Fahrenheit 451. You’re dealing with book burning, a very serious subject. You’ve got to be careful you don’t start lecturing people. So you put your story a few years into the future and you invent a fireman who has been burning books instead of putting out fires—which is a grand idea in itself—and you start him on the adventure of discovering that maybe books shouldn’t be burned.