This book is actually an ingenious and provoking exploration of some possible implications of uploading a human mind into a computer.

Starting from the premise that this is possible, and observing that computation is an ideal, abstract thing, unbound in time, Egan disables by stages our intuitions about the relations between the experience of the reluctantly uploaded protagonist, Paul, and the computations that realise him.

At first, the 'time resolution' - the frequency at which the brain-states are computed - is slowed. Then the computations are performed in reverse order; then shuffled; and for the finale, run in parallel on a seti at home-like distributed network. Paul, whose experiences are realized by these computations, notices nothing out of the ordinary, no change in the order of his perceptions, no gaps in his sense of time.

Prompted by these experiences, and struggling to explain the continuity of his experience in the face of the temporal and spatial discontinuity of its calculation, Paul theorises:

What characterises one point in space, for one instant? Just the values of the fundamental particle fields, just a handful of numbers. Now, take away all notions of position, arrangement, order, and what's left? A cloud of random numbers. [...] There's no reason to believe that the pattern we've found is the only coherent way of ordering the dust.

[...] What was I made of when the processors weren't describing me? Well ... it's a big universe. Plenty of dust to be me in between descriptions.

In the second half of the book the ramifications of this 'dust theory' (which first surfaced, I'm told, in Egan's short fiction: Dust) help Paul to implement and inhabit quite the most elegant and efficient computer architecture in all of sci-fi.

There's also a charming subplot about a cute artificial universe, the 'Autoverse' - modeled as a cellular automaton - with its own toy physics, chemistry, and biochemistry, which finally comes back to bother its creators.

Perhaps it's true, from a literary point of view, that the novel loses its direction a little towards the end. But still, it's always stimulating, bursting with ideas, plot, and in-depth imagination. Egan has thought his way far into this strange territory, and though the book is written from a strong-AI perspective, it presents noteworthy questions for anyone interested in strong-AI views, pro or con, to ponder.

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