[Published in Philosophical Quarterly (2003) Vol. 53, No. 211, pp. 243-255.
(First version: 2001)]
But the scenario depicted in the movie
is ridiculous: human brains being kept in tanks by intelligent
machines just to produce power.
Perhaps its most startling lesson is that there is a significant probability that you are living in computer simulation.
I mean this literally:
What grounds could we have for taking this hypothesis seriously?
Before getting to the gist of the simulation argument, let us consider some of its preliminaries.
One of these is the assumption of
“substrate independence”. This is the idea that conscious minds
could in principle be implemented not only on carbon-based
biological neurons (such as those inside your head) but also on some
other computational substrate such as silicon-based processors.
But ultimately, what allows you to have conscious experiences is not the fact that your brain is made of squishy, biological matter but rather that it implements a certain computational architecture. This assumption is quite widely (although not universally) accepted among cognitive scientists and philosophers of mind.
For the purposes of this article, we
shall take it for granted.
Doing so would require very powerful hardware that we do not yet have. It would also require advanced programming abilities, or sophisticated ways of making a very detailed scan of a human brain that could then be uploaded to the computer.
Although we will not be able to do this in the near
future, the difficulty appears to be merely technical. There is no
known physical law or material constraint that would prevent a
sufficiently technologically advanced civilization from implementing
human minds in computers.
Furthermore, we can establish lower bounds on how powerful the computers of an advanced civilization could be. Technological futurists have already produced designs for physically possible computers that could be built using advanced molecular manufacturing technology.
The upshot of such an analysis is that a
technologically mature civilization that has developed at least
those technologies that we already know are physically possible,
would be able to build computers powerful enough to run an
astronomical number of human-like minds, even if only a tiny
fraction of their resources was used for that purpose.
But all that
this shows, so far, is that you could never be completely sure that
you are not living in a simulation. This result is only moderately
interesting. You could still regard the simulation hypothesis as too
improbable to be taken seriously.
Instead, it shows that we should accept as true at least one of the following three propositions:
Each of these three propositions may be
prima facie implausible; yet, if the simulation argument is correct,
at least one is true (it does not tell us which).
Then a significant fraction of all species at our level of development eventually becomes technologically mature. Suppose, further, that (2) is false, too.
Then some significant fraction of these species that have become technologically mature will use some portion of their computational resources to run computer simulations of minds like ours.
But, as we saw earlier, the number of
simulated minds that any such technologically mature civilization
could run is astronomically huge.
In other words, almost all minds like yours, having the kinds of experiences that you have, would be simulated rather than biological.
Therefore, by a very weak principle of
indifference, you would have to think that you are probably one of
these simulated minds rather than one of the exceptional ones that
are running on biological neurons.
is relatively straightforward. For example, maybe there is some
highly dangerous technology that every sufficiently advanced
civilization develops, and which then destroys them. Let us hope
that this is not the case.
One can imagine various reasons that may
lead some civilizations to forgo running simulations, but for (2) to
obtain, virtually all civilizations would have to do that. If this
were true, it would constitute an interesting constraint on the
future evolution of advanced intelligent life.
is correct, you are almost certainly now living in computer
simulation that was created by some advanced civilization. What kind
of empirical implications would this have? How should it change the
way you live your life?
To a first approximation, if you thought
you were in a simulation, you should get on with your life in much
the same way as if you were convinced that you are living a
non-simulated life at the bottom level of reality.
To the extent that you think that you understand the motives of the simulators, you can use that understanding to predict what will happen in the simulated world they created.
If you think that there is a chance that the simulator of this world happens to be, say, a true-to-faith descendant of some contemporary christian fundamentalist, you might conjecture that he or she has set up the simulation in such a way that the simulated beings will be rewarded or punished according to christian moral criteria.
An afterlife would, of course, be a real possibility for a simulated creature (who could either be continued in a different simulation after her death or even be “uploaded” into the simulator’s universe and perhaps be provided with an artificial body there).
Your fate in that afterlife could be made to depend on how you behaved in your present simulated incarnation.
Other possible reasons for running
simulations include the artistic, scientific or recreational. In the
absence of grounds for expecting one kind of simulation rather than
another, however, we have to fall back on the ordinary empirical
methods for getting about in the world.
But if they choose to reveal themselves, they could certainly do so. Maybe a window informing you of the fact would pop up in front of you, or maybe they would “upload” you into their world.
Another event that would let us conclude with a very high degree of confidence that we are in a simulation is if we ever reach the point where we are about to switch on our own simulations.
If we start running simulations, that would be very strong evidence against (1) and (2).
That would leave us with only (3)...