Shikasta, the first in what would become five science fiction novels by Doris Lessing, begins with a journey in to the pre-history of the planet Rohanda. Johor is an emissary of the empire of Canopus, sent to help the development of Rohanda. Johor's journey is made through six "zones", levels of spiritual existence each becoming more solidly material, until he arrives at Rohanda itself, which we come to suspect is in fact our own Earth.

Rohanda is the proudest achievement of the Canopean empire, a paradise world where humankind's early ancestors have been nurtured into a utopian civilisation, with some gentle interference from the massively more advanced Canopeans. But soon after Johar's first journey to our world, Rohanda will become Shikasta, "the broken", a fallen world, sheared from the "galactic Lock" that has brought it peace and development, and exploited by the evil empire of Shammat.

Doris Lessing is widely recognised as one of the great literary novelists of the 20th century. The Golden Notebook and The Grass Is Singing underpin a reputation that won their author the Nobel prize for literature in 2007. But it was the Canopus in Argus sequence, identified by Lessing as her most important work, that the Nobel committee recognised when describing their author as "the epicist of the female experience".

Lessing's departure into science fiction in the late 1970s raised some predictably snobbish responses from literary critics, one writing that she "propagandises on behalf of our insignificance in the cosmic razzmatazz". But more serious thinkers like Gore Vidal recognised the epic scope of Shikasta, placing it in context with John Milton's Paradise Lost as an allegorical work on themes of religion and the divine.

Shikasta's allegorical meaning also makes it an awkward fit within science fiction, a genre that tends to take its spaceships and alien visitations literally. But Lessing achieves in Shikasta exactly the thing that is so often missing from SF, and in so doing demonstrates what a strong literary imagination can achieve in a genre that, for all its big ideas, rarely manages to touch real human experience.

In no literature is the faith in technological progress more solid than in science fiction. So it's a mark of Lessing's genius that she encapsualted the genre's most basic philosophical tenets and turned them, almost effortlessly, back on themselves. Shikasta reinvents SF as a literature of mystical speculation, exactly the philosophy the genre itself so often rejects.

Lessing does take care to emphasise the scientific basis of her speculation throughout the novel. Johor's missions to Rohanda are a matter of guided evolution, not divine intervention. The first human cities established by Canopus are products of pure mathematics, taking base geometrical shapes like the Square, Circle and Oval as their pattern. Beyond these cities, nature exists in a state much like the Garden of Eden.

Nonetheless, Lessing's novel is shot through with mystical and religious ideas. Johor journeys to Rohanda through zone 6, a metaphysical realm crowded with the souls of the dead awaiting rebirth, much religions various versions of purgatory. Later, Johor himself reincarnates as an infant human and grows to full adulthood as one George Sherban, a messiah figure who will help humanity through the coming apocalypse.

The sweep of Lessing's vision in Shikasta is staggering, beginning at the prehistory of Earth, taking in galaxy-spanning vistas, and homing in on details of modern society. Lessing's portrait of a divided world, forever at war with itself, is as relevant in our post-911 world of mass digital surveillance as ever. We are, as Lessing describes the basic sickness of Shikasta, a world with "too little Spirit of We Feeling".

Shikasta charts the fall of mankind, from a state of utopian paradise to the hellish conflicts of the modern world. For many readers the portrait of the modern world as a place fallen from a much higher ideal is a difficult one to accept. Our modern technological narrative is one of progress. In the words of another great SF writer, Terry Pratchett, who would not rather be a rising ape than a falling angel? In Shikasta, Lessing challenges us to consider that we may be both.