What is Science Fiction?

Over at Yahoo! Questions, someone asked, "What do YOU think science fiction is? Do you think it could be a prediction of the future?"

I posted this answer:

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My own definition is this: Science fiction is the mainstream literature of a plausible alternative reality. That is to say, it is stories told as if to people already familiar with the story's milieu, but that milieu is one the author has contrived but could exist (or, in the case of alternate history stories, could have existed). If a story is set on a Martian colony in the year 3000 A.D., it's told as if the reader is already a member of that colony, or at least lives in a reality in which such a colony is well known (just as a mainstream novel for an American audience might in fact be set in modern Australia).

This is part of the special joy of science fiction: the reader, of course, isn't actually familiar with the milieu, and loves the process of picking up clues, artfully salted by the author, as to what the nature of the setting really is. But the skilled SF author will not stop to flat-out explicate things his or her reader, were they really contemporaries of the story's characters, would actually know.

I use the phrase "alternate reality," rather than simply calling SF "the mainstream literature of the future," in part because of the large body of work known as "alternate history" or "parallel-worlds stories," which are usually considered part of science fiction.

My definition seeks to define SF as a storytelling mode, rather than by listing an arbitrary series of tropes (spaceships, time travel, aliens), and I think it does a good job of accurately encompassing most work in the field. Of course, there are always exceptions, but I've found this definition has served me well over the years.

To your second point, yes, science fiction might sometimes predict the future, but that is rarely its intent. Just as often, as Ray Bradbury has said, it's job is to PREVENT the future. If accurate prediction were the criterion of good SF, we'd have to say that George Orwell's NINETEEN EIGHTY-FOUR was an abysmal failure because the real year 1984 turned out nothing like his prediction. But in fact Orwell's novel was a resounding success because its warning call helped us to keep the future it portrayed from becoming reality.

The Robert J. Sawyer Web Site