Official author synopsis, published excerpt and author's retrospective FAQ checked. Complete novel pagination has not been reviewed for this edition.
Cabinet / close reading and mathematical apparatus
A universe cannot supply its own past
Greg Egan's Permutation City makes a simulated world depend on one set of rules, then asks whether its inhabitants need the machinery running at all.
He can cancel the experiment.
Paul Durham's Copies have the memories of the flesh-and-blood Paul, yet they do not inherit his willingness to remain inside the experiment. Some demand to be shut down. In the novel's opening this is neither a programming error nor a philosophical abstraction: the model speaks from a position its original did not fully imagine.
The environment is already economical. A simulated room needs detailed illumination where the Copy looks, while unseen surfaces receive less computation. The method anticipates familiar rendering strategies, but it does not explain whether the simulated inhabitant feels anything. Rendering convincing surroundings and reproducing a mind are separate engineering claims.
Even a perfect functional model would not by itself answer which person owns the decision to terminate. Consent by the original before a scan is a different act from consent by the later Copy. That difference survives any improvement in graphics or computing speed.
Maria begins with laws.
Maria Deluca works in the Autoverse, a virtual world specified by local computational rules. Rather than draw scenery and animate its inhabitants, she is asked to construct the seed of a biosphere that could develop under those rules. The scientific wager is about sufficiency: can the proposed physics admit stable chemistry, persistence and evolution?
The premise demands far more than geometric plausibility. A convincing biosphere would require mechanisms for acquiring resources, maintaining boundaries, reproducing with inherited variation and remaining viable through disturbances. The book's project is an invented one; this page does not claim that any corresponding biosphere has been built.
The computational cost is part of the fiction. Egan's synopsis notes that an Autoverse large enough to sustain Maria's planned biosphere would exceed the computing resources available in the story. That obstacle motivates the story's metaphysical ambitions, which should not be mistaken for a known method of avoiding computation.
Apparatus / A finite universe with an imperfect memory
Every state has a countable past.
Select a local rule. Then choose a present configuration of eight cells. The machine enumerates all 256 earlier configurations and counts how many could have produced the selected one in a single step. Below it, the selected state advances for eighteen generations.
Simple additive structure can generate many unreachable states on this finite ring.
No predecessor
Across all 256 present states
History from the selected state
Figure 01. A finite state becomes a predecessor problem. All eight-cell binary states are checked directly, without sampling. Figure 02. Subsequent generations under the chosen local rule; time increases downward. Each row is a calculated state.
Finite ring with eight binary cells. The exact counts depend on the selected rule and boundary conditions. This does not implement the Autoverse or settle the Dust Theory.
An unreachable beginning is not a unique history.
A Garden-of-Eden configuration is a state that cannot be reached from any earlier state under a given cellular-automaton rule. Durham tries to use an unreachable beginning to establish something about the history of the world he inhabits. The appeal is understandable: if a state could not have followed an earlier one, perhaps the resulting world has an especially definite origin.
The argument runs into a different difficulty. Some update rules have multiple possible predecessors for a later state. A world can begin with a state that has no predecessor and still evolve into one that does not uniquely reveal its own past. Egan discusses this objection explicitly in his 2009 addendum, crediting Yair Glasner for pointing it out.
The apparatus below lets readers inspect this distinction on a finite ring. Every predecessor is enumerated. The model demonstrates a fact about finite state machines; the Garden-of-Eden theorem for infinite cellular automata addresses a different domain. The calculation says nothing about conscious universes.
Three claims, three kinds of evidence
The generated world behaves according to its stated rules.
Mathematical
Reproduce the update and enumerate predecessors in a fully specified finite model.
True of this example; no inference about biological richness.
A functional Copy behaves like the original person.
Empirical / hypothetical
Would require validated biological models and longitudinal behavioral comparisons.
No completed whole-person emulation or identity test is documented here.
The Copy has conscious experience irrespective of substrate.
Philosophical
Would require a defensible account of subjective experience and why implementation does or does not matter.
The cellular-automaton computation offers no answer. Egan questions the pure Dust Theory himself.
An evolved population would have to live through it.
Maria's seed is imagined as the beginning of a long artificial evolutionary history. An engineer might regard the successive generations as search steps toward a more complex system. The inhabitants, if conscious, would have their own lives rather than the engineer's objective function.
Egan later called attention to a serious omission in the novel: the suffering that could be imposed by trying to evolve intelligent creatures through vast numbers of births and deaths. His FAQ makes this an ethical objection, not a technical obstacle that faster hardware could resolve.
Any real experiment seeking artificial life needs a threshold for discussing possible welfare and a plan to stop before uncertain computational success is used to justify potentially immense harm. Current toy automata offer no reason to assign subjective experience to the squares shown below.
Permutation City in the author's own words
The primary reference is the 1994 book. These links document what we checked in this working edition. The model and philosophical comparisons are our editorial apparatus.
- Author's primary sourceGreg Egan / Permutation City ↗
Author's synopsis, Autoverse, Copies and bibliographic history.
- Author's primary sourceGreg Egan / published excerpt ↗
Official prologue featuring a Copy's situation and the rendering of its environment.
- Author's commentary, revised 2009Greg Egan / Dust Theory FAQ ↗
Author's retrospective comments on temporal recomputation, the Dust Theory, ethics and the Garden-of-Eden argument.