How can one resist a back-of-book blurb that reads, "Beyond space, time, eternity - the ultimate creation dream"? Quite easily, actually. But when the author is Greg Egan, you'll discover that a blurb like that is not as much of an exaggeration as it seems.

Permutation City has many strengths, not least of which is Egan's investigation of the fascinating possibilities offered by Virtual Reality (VR). At the start of the book, a character wakes up as a digital copy of himself in a tangible 3D world with all five his senses functioning. He can even conjure up a menu out of empty air to transform his surroundings or call up the non-digital version of himself. Using this scenario, Egan skillfully leads us to question "Real Reality" and our place in it. (I think Philip K. Dick, Science Fiction's master of Reality-Called-Into-Question, would have approved.)

From this auspicious start Egan goes on to present storylines which, while they intersect, do not mesh particularly tightly. Still there's ample material for that famous SF "sense of wonder". Take, for instance, the incredible TVC. The TVC is the vehicle for "Permutation City", a potentially infinite, ever-malleable hyperspace heaven, the building blocks of which reproduce themselves into ever-multiplying units of space and processing power.

But here it gets rather fantastic: Egan makes a case for packing off VR copies of human beings into a computer network, letting it run for, say, 20 seconds, then switching off the hardware while the TVC universe just keeps running by virtue of its own internal logic. Impressive? Undoubtedly. Plausible? Well… I don't think plausibility was the point for Egan.

Greg Egan is perhaps less concerned with creating an objective SF world underpinned by plausible physics than he is in creating challenging and exciting spin-off worlds. This is, of course, perfectly in keeping with the idea that in VR anything can change, in any way, at any time - including the constant transformation of a VR person's environment and even that person's own identity. It is the stuff that dreams are made of, but always with some link to a reality outside the Virtual.

It is this link to an outside reality, a far-away place of which traces can be found in the very fabric of the Virtual, that keeps Permutation City from floating off into unmitigated Fantasy.

I believe this is at least one of the reasons why Egan uses the words "Permutation City" in an anagram-poem at the beginning of his novel. Here are some of the lines: "To trace impunity, I trace an outcry, I pin my taut erotic art to epic mutiny". The 15 letters of the book title slip and slide, fleetingly reorganising themselves to form ephemeral, shape-shifting visions which nevertheless can always be traced back to their origin.