I'd also wager that "I don't know Timmy" is more thematically related. I feel most of the discussion in this thread glosses over what is most unsettling about Permutation City. It isn't just a book about what it could be like to be a simulated mind, it's most deeply about exploring the disquieting metaphysical consequences of computable minds. I can't think of a story that has as thoroughly scattered my basic grasp of reality as this one. Only Blindsight even begins to comes close.
In "I don't know Timmy" there's a sequence that goes:
"Well, we can't exactly turn it off."
"Why not?" asked Tim, halfway to the door, then stopped mid-stride and stood still, realising.
"Oh."
But you can turn it off without consequence and Permutation City explores the disturbing implications of why thoroughly (with a deus ex machina ending to save causal physics, as expected of an Egan story, physics is what has plot armor).
> I can't think of a story that has as thoroughly scattered my basic grasp of reality as this one. Only Blindsight even begins to comes close.
Absolutely! Very few books have also basically changed my philosophical outlook on something as much as this has (I had the seeds of the idea before the book, but the book really cemented specific concepts around computation and thought/consciousness/identity even mean).
Another book that is fairly different, but has had a big impact on some of my views of things, is the Three Body Problem trilogy (specifically the second and third book).
> But you can turn it off without consequence and Permutation City explores the disturbing implications of why thoroughly
Can you turn it off? The entire universe is 100% deterministic and the "stack" of universes in question is based on the same seed data. So if you decide to turn the n+1 simulation off, the same decision will be made in the n-1 universe. The simulation isn't running on some self-replicating automaton like in Permutation City. Of course the universe being 100% deterministic also poses the question if you can "decide" to do anything, since all decisions are already made.
The point is that if the universe is entirely deterministic then it is entirely described by its starting conditions: from the point of view of entities within it, it makes no difference if you run the simulation or not, they exist from their own perspective. Running the simulation only matters to you. So in the case where you have recursive simulations of the same universe like in the story, even if you turn of your simulation, implying the universe 'above' turns off theirs, you will continue to exist just fine, as far as you can see. That is the core idea of permutation city: they set up a simulation containing themselves and then don't bother to continue to run it: it doesn't matter to the simulated version of themselves (of course, you can ask the question whether they would need to set up the simulation in the first place. And then things get more crazy).
In "I don't know Timmy" there's a sequence that goes:
"Well, we can't exactly turn it off."
"Why not?" asked Tim, halfway to the door, then stopped mid-stride and stood still, realising.
"Oh."
But you can turn it off without consequence and Permutation City explores the disturbing implications of why thoroughly (with a deus ex machina ending to save causal physics, as expected of an Egan story, physics is what has plot armor).