The Bactra Review: Occasional and eclectic book reviews by Cosma Shalizi   40

Permutation City

by Greg Egan

London: Millennium, 1994

Simulated Leibniz

Permutation City is a gripping novel about cellular automata and abstract computation, about consciousness and the nature of the self, about space, time, causality, and the constitution of all possible worlds. I admit I came to it prepared to be profoundly unimpressed by the writing, but by the end of the second chapter I was well and truly hooked, and then Egan began pulling out all the metaphysical stops. Imagine Philip K. Dick channeling Marvin Minsky channeling Bertrand Russell channeling Leibniz with a few hundred micrograms of LSD tossed in at some point in the chain --- imagine a novel which can use the phrase ``the economy of ontology,'' not just without grinning or blushing, but perfectly convincingly --- and you'll have some idea of what Permutation City is like.

Leibniz thought that this actual world was singled out from all other possible worlds being being the fullest, the one with more compossible elements than any other. But why single out one possible world as actual at all? All of them are perfectly good logical possibilities, and from the inside one looks as real as another. (This is true even of the worlds where everyone has an unshakable conviction that they are not real.) You don't even need a separate universe for each one; all you need are events, sheer unordered chaos. Every internally consistent ordering of events into a space-time will look as good from the inside as every other; all the possible worlds exist at once, interpenetrating and intermingling, ``assembling themselves from the dust.'' This is what Egan's characters call ``the hypothesis of the dust,'' and proceed to exploit.

Accepting, for the sake of the novel, that consciousness is just a matter of a certain kind of computation, it can be perched atop an arbitrarily high tower of simulation. You can start with physical computers, implement a Turing-equivalent, indefinitely growing cellular automaton on it, and then implement a consciousness as a program running on a computer constructed in the cellular automaton. You do this so that when the physical computers are turned off, your consciousness (already copied into an abstract, programmatic form) is already firmly embedded in an ever-growing CA world, which is perfectly self-consistent and perfectly stable, and so, by the hypothesis of the dust, will continue regardless of whatever happens to the universe it was spawned from. This makes no sense, of course, since by that self-same hypothesis of the dust that universe already exists, whatever you do in this one; but human minds are not equipped to follow stories told sub specie aeternatis, and Egan is a good enough story-teller to carry the plot over this obstacle by sheer force. Then things begin to get strange.

It took me about half a day to argue myself out of the hypothesis of the dust. I'm still not completely certain it's altogether wrong. I am certain that I should have read this book when it came out, four years ago. Do so at once.

310 pp.
Cellular Automata / Computers and Computing / Mind, Consciousness, etc. / Science Fiction
In the US, Harper Mass Market Paperbacks, US$4.99, ISBN 006105481X; in the Commonwealth, London: Millennium, L4.99, Aus. $12.95, ISBN 1-85798-218-5
20 April 1998