On a perfectly normal day in May - a Wednesday to be precise - with no dragons in the skies (or none that I notice) and no curious mists swirling around Hillhead Tube Station, I travel to Glasgow University to discuss new possibilities in impossibility with Robert Maslen.

Professor Maslen is an expert in 16th- and 17th-century literature who has written scholarly works on Shakespeare's comedies and Sir Philip Sidney's Apology For Poetry. He can also tell you all you need to know about Jacobean revenge tragedies. And, oh yes, he quite likes fantasy fiction.

In fact he likes fantasy fiction so much that later this year he will oversee the first MLitt in fantasy fiction. That's the first in the world, just so we're clear (I can't vouch for any other world obviously). In a way it seemed an obvious thing to do, he says as we sit in his office. Maslen already runs an undergraduate course in fantasy which is open to 40 students a year. The demand, he says, is there.

"For a long time it was embarrassing to acknowledge that you loved fantasy," he admits. "But I put it up on the internet several years ago that fantasy was my obsession and people started coming to me. I've had PhD students coming to me in very large numbers interested in fantasy topics."

Perhaps that should be no surprise. George R Martin's Game Of Thrones books are now the subject of a hugely successful TV series and Susanna Clarke's novel Jonathan Strange & Mr Norrell has been turned into prime-time Sunday night fare by the BBC. Tolkien's Lord Of The Rings remains a phenomenon. And over the last few years authors such as China Mieville and Jeff Vandermeer have been promoting a subgenre which they have called "the new weird", one that rejects some of the more traditional, romantic tropes the genre abounds in (ie, it's not all dungeons and dragons). In short, we are in a golden age for fantasy, you might say. In fact Professor Maslen does say.

It seems a good time, then, to talk about it. To ask what it is and what it's for and why we should we pay attention. What is fantasy's story and why do so many people have a problem with it?

The definition is the easy bit, according to Maslen. It's the "fiction of the impossible," he says. "In other words I think it is necessary that you know what you're reading doesn't happen now, could never happen now and could never really happen in the future."

He uses Kafka's short story Metamorphosis as an example. "A man turns into a bug. We know full well that could never happen. But at the same time what the story is interested in is not the science of how he might have metamorphosed, but the fact that he's turned into a beetle and how his family reacts to it."

Fantasy remains a loaded word, Maslen admits - in both academic circles and to large portions of the general public. Perhaps because it is something we often first encounter in childhood and it bears the weight of that. We apply the term "childish" to it.

Maslen is happy to say he never grew out of the form. "I believe I simply found my adult concerns were as well reflected in the adult fantasies that I read as they had been in the children's fantasies and, of course, I continued to find new things in the children's fantasies that I used to read."

A confession: I quite like fantasy too. I grew up on Robert E Howard's Conan The Barbarian stories via Marvel Comics, devoured Michael Moorcock's Runestaff novels as a teenager and, in recent years, have read both Mieville and Vandermeer. I don't like the quasi-medievalist stuff and can't be doing with Tolkien at all, but that's just a matter of taste.

At the same time, though, I'm not sure I could articulate a simple defence for the form. It's easy to do so for related genres. Science fiction is a way in which we can contemplate our relationship with technological and social change and address our place in the cosmos. Horror, meanwhile, offers us a way to examine our fear of death and explore the return of the repressed. But what does fantasy allow?

It's that notion of impossibility that's key, Maslen reckons. Fantasy questions all our definitions of cause and effect, he argues. "We have narratives to tell ourselves about how the world works. One such narrative is the language of science. But in fact much of our lives we spend not really knowing what's making things happen at all.

"Fantasy steps into that breach and enables us to think of alternative ways of telling our own story which are entirely different to the ones we have been taught."

What might this mean in practice? Maslen takes a detour into his other area of expertise, the Enlightenment.

"If you look at the Reformation it could be said to be a religious revolution. One of the things the revolutionaries did was to rewrite the past immediately and to start to find a continuity from our past into their present which enabled them to suggest they had a background to their movement, that it had been going on as the authentic form of Christianity right through the centuries up to that moment. It seems to me that you could argue that some kinds of fantasy do exactly that. They look to unearth the unspoken, the forgotten, the lost."

Tolkien is a good example, he says. The author of The Lord Of The Rings was an Anglo-Saxon scholar, an area in which there are few very few texts left. So in a way he started making a new one, Maslen suggests, "building a gigantic mythology of his own to replace all the lost mythologies which he has to work out by reading a poem like Beowulf.

"And of course you can see how that could be adapted to radical purposes. Like recovering the voices of women who have been lost in history, or recover the utopian impulses that people have had that come to nothing. Those kinds of possibilities are open to fantasy."

The MLitt course will cover the Gothic novels of the 19th century, the works of George MacDonald and Lord Dunsany, Tolkien of course, Mervyn Peake's Gormenghast trilogy and Ursula LeGuin's Earthsea trilogy (fantasy writers do like their trilogies) and many points between and beyond.

Like the undergraduate course, it will seek to historicise fantasy writing. Maslen cites the example of Lord Dunsany, a man who saw service in the Boer War and the First World War, who was injured by the rebels during the Easter Rising in 1916 and yet chose to write such novels as The King Of Elfland's Daughter. Asking why seems an interesting question.

At which point maybe we should reverse the line of enquiry here and ask why we think realism is so great anyway? Why do we consider it the highest literary form?

"I think it's something to do with academia and the teaching purpose," says Maslen. "I suppose in a way it's a moralistic urge, a sense that reading ought to teach you something. I think in universities and schools it was felt to justify the teaching of literature you had to demonstrate that it was instructive."

And the truth is, he points out, that many literary and academic critics of the form fail to recognise that most major writers of the 20th century wrote fantasy at some point or other. "I always used Woolf's Orlando as an example, or Orwell's Animal Farm. You can look at how magic realism has been deployed to consider matters of race and nation by people like Toni Morrison and Salman Rushdie. You might look at how gender issues have been explored by Doris Lessing or Angela Carter. You could look at the way sexual desire has been explored in fantasy by writers like Samuel Delaney. The variety of what you can do with it is fantastic."

And maybe there's a bigger idea lurking under the surface. Maybe we should be asking why don't more people write fantasy when the world is itself so prone to it. How normal is normal anyway given the history of the last 100 years? Indeed you might argue that the abhorrent political philosophy of Nazism was based on a Wagneresque fantasy.

"The shock and horror at the rise of Hitler and Nazism in the 1930s was that it was a form of fantasy that people tried to make real," suggests Maslen. "Interestingly, the attack on Nazism was very often marshalled around various forms of fantasy and science fiction, not all of it terribly well known but a lot of it tremendously interesting. There are writings like Joseph O'Neil's Land Under England, which is about a man who goes looking for his father who has fallen down a hole under Hadrian's Wall to an underground civilisation that's left over from the Roman colonisation of Britain. The people there practise mind control and march around in a form of goosestep and are very clearly fascistic. So Nazism as an imaginative form is responded to in imaginative form."

When I leave Glasgow University and walk back to the subway, there are still no dragons or mists in sight. But all around me are invisible electronic signals. Above my head satellites circle the earth and nearby, no doubt, physicists are still trying to make sense of quantum mechanics. The world is not as we see it. Fantasy fiction knows this already.

For more details on the MLitt course, visit www.glasgow.ac.uk/pg/fantasy