Before the term "science fiction" appeared, in America in the 1930s, during the golden age of bug-eyed monsters and girls in brass brassieres, stories such as HG Wells' The War of the Worlds were called "scientific romances". In both terms - scientific romance and science fiction - the science element is a qualifier. The nouns are "romance" and "fiction", and the word fiction covers a lot of ground.

If you're writing about the future and you aren't doing forecast journalism, you'll probably be writing something people will call either science fiction or speculative fiction. I like to make a distinction between science fiction proper and speculative fiction. For me, the science fiction label belongs on books with things in them that we can't yet do, such as going through a wormhole in space to another universe; and speculative fiction means a work that employs the means already to hand, such as DNA identification and credit cards, and that takes place on Planet Earth. But the terms are fluid. Some use speculative fiction as an umbrella covering science fiction and all its hyphenated forms - science fiction fantasy, and so forth - and others choose the reverse.

I have written two works of science fiction or, if you prefer, speculative fiction: The Handmaid's Tale and Oryx and Crake. Here are some of the things these kinds of narratives can do that socially realistic novels cannot do.

· They can explore the consequences of new and proposed technologies in graphic ways, by showing them as fully operational. We've always been good at letting cats out of bags and genies out of bottles, we just haven't been very good at putting them back in again. These stories in their darker modes are all versions of The Sorcerer's Apprentice: the apprentice finds out how to make the magic salt-grinder produce salt, but he can't turn it off.

· They can explore the nature and limits of what it means to be human in graphic ways, by pushing the envelope as far as it will go.

· They can explore the relationship of man to the universe, an exploration that often takes us in the direction of religion and can meld easily with mythology - an exploration that can happen within the conventions of realism only through conversations and soliloquies.

· They can explore proposed changes in social organisation, by showing what they might actually be like for those living within them. Thus, the utopia and the dystopia, which have proved over and over again that we have a better idea about how to make hell on earth than we do about how to make heaven. The history of the 20th century, where a couple of societies took a crack at utopia on a large scale and ended up with the inferno, would bear this out. Think of Cambodia under Pol Pot.

· They can explore the realms of the imagination by taking us boldly where no man has gone before. Thus the space ship, thus the inner space of the hilarious film Fantastic Voyage, the one where Raquel Welch gets miniaturised and shot through the blood stream in a submarine. Thus also the cyberspace trips of William Gibson; and thus The Matrix, Part 1 - this last, by the way, an adventure romance with strong overtones of Christian allegory, and therefore more closely related to The Pilgrim's Progress than to Pride and Prejudice.

More than one commentator has mentioned that science fiction as a form is where theological narrative went after Paradise Lost, and this is undoubtedly true. Supernatural creatures with wings, and burning bushes that speak, are unlikely to be encountered in a novel about stockbrokers, unless the stockbrokers have been taking a few mind-altering substances, but they are not out of place on Planet X. The form is often used as a way of acting out the consequences of a theological doctrine. The theological resonances in films such as Star Wars are more than obvious. Extraterrestrials have taken the place of angels, demons, fairies and saints, though it must be said that this last group is now making a comeback.

We want wisdom. We want hope. We want to be good. Therefore we sometimes tell ourselves warning stories that deal with the darker side of some of our other wants. As William Blake noted long ago, the human imagination drives the world. At first it drove only the human world, which was once very small in comparison to the huge and powerful natural world around it. Now we're close to being in control of everything except earthquakes and the weather.

But it is still the human imagination, in all its diversity, that directs what we do with our tools. Literature is an uttering, or outering, of the human imagination. It lets the shadowy forms of thought and feeling - heaven, hell, monsters, angels and all - out into the light, where we can take a good look at them and perhaps come to a better understanding of who we are and what we want, and what the limits to those wants may be. Understanding the imagination is no longer a pastime, but a necessity; because increasingly, if we can imagine it, we'll be able to do it.