A Computational Copernican Principle
In most tales of simulated worlds, the tissue of plausibility becomes thinnest at the interface between levels of reality. I can believe (just barely!) in a civilization that exists only as a computer program. Where my suspension of disbelief becomes least willing is in the crossing over between a physical world and an algorithmic one. In movies the leap is often made by putting on a skullcap studded with electrodes or by plugging a cable into your spinal cord. It seems to me there is a fundamental category violation here. I am made of atoms and molecules. How could I enter a world of bits and bytes? (But maybe that's what all simulated creatures say.)
Moravec, in his parable of the Cellticks, handles this issue more carefully than other authors. His programmer never steps into the gridlike space of the cellular automaton. The Celltick program does become an inhabitant of the programmer's world, but not by any kind of magic metamorphosis. The transmigration of souls happens in easy technological stages. First, microphones and television cameras are attached to the Cellticks' computer to give them sensory experience; then the computer is made mobile, so that they can explore on their own.
The most intriguing part of Moravec's fantasy deals with the crucial moment of discovery, when the Cellticks first learn their true ontological status. They take a scientific approach, studying the transition rules that constitute the laws of nature in their universe. "Once in a long while the transition rules are violated, and a cell that should be on goes off, or vice versa . . . . After recording many such violations, the Cellticks detect correlations between distant regions and theorize that these places may be close together in a larger universe." From this slender clue they learn the structure of the computer that is running the program that creates their world, and they decipher its machine language. We would call this process reverse engineering, but to the Cellticks it is physics.
It seems significant that malfunctions have a role in the Cellticks' cosmological investigation. In a properly functioning computer, a program cannot learn anything about the hardware on which it is running. True, the program might think it has learned something. It might go digging through read-only memory and find buried there the telltale markers of an Apple II computer. But the ability of one computer to emulate another makes such digital archeology untrustworthy. The Apple II might be an emulation running on an IBM PC, or a HAL 9000. If the emulators are written correctly, they can reproduce even the most obscure quirks and bugs of the target hardware. Unless you get lucky and spot a glitch in the Matrix, no program will detect the fraud.
Once you begin to take such ideas seriously, the situation goes from bad to worse in a hurry. Consider this: If a simulation is complete enough to have some kind of intelligent entities within it, then those entities could also build computers to simulate worlds, which could include still more computers and simulations of their own. In this tower of simulations, where would our world fit? To answer that question it seems best to invoke a computational Copernican principle. Just as the earth is unlikely to lie at the center of the universe, our level of simulation is unlikely to lie at either the very top or the very bottom of the tower.
This principle can be followed along a curious trail of further arguments. Although we might not be directly aware of any levels of simulation above us, we ought to know about those below us, since they are our own creations. But no such levels exist; we have not (yet) created any artificial civilizations. Thus we seem to be at the very bottom of the tower, which is unlikely, and so it seems safe to assume we are real flesh and blood after all. But this reassuring chain of reasoning has a dark side. If we ever do construct a simulated world rich enough in resources that its inhabitants can create their own simulated worlds, then on that basis alone we might have to conclude that we ourselves are a simulation.