by Nick Bostrom
Times Higher Education
Supplement, May 16, 2003
[Published in Philosophical Quarterly
(2003) Vol. 53, No. 211, pp. 243-255.
(First version: 2001)]
Nick Bostrom is
Professor at Oxford University, Faculty of Philosophy &
James Martin 21st Century School. His simulation
argument was first published in The Philosophical
Quarterly. A preprint of the original paper is available
The Matrix got many otherwise not-so-philosophical minds ruminating
on the nature of reality.
But the scenario depicted in the movie
is ridiculous: human brains being kept in tanks by intelligent
machines just to produce power.
There is, however, a related scenario that is more plausible and a
serious line of reasoning that leads from the possibility of this
scenario to a striking conclusion about the world we live in. I call
this the Simulation Argument.
Perhaps its most startling lesson is
that there is a significant probability that you are living in
I mean this literally:
if the simulation hypothesis is
true, you exist in a virtual reality simulated in a computer
built by some advanced civilization. Your brain, too, is merely
a part of that simulation. 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.
Of course, the computers we have today are not powerful enough to
run the computational processes that take place in your brain. Even
if they were, we wouldn’t know how to program them to do it. 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.
Given substrate independence, it is in principle possible to
implement a human mind on a sufficiently fast computer.
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.
Our second preliminary is that we can estimate, at least roughly,
how much computing power it would take to implement a human mind
along with a virtual reality that would seem completely realistic
for it to interact with.
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.
If you are such a simulated mind, there might be no direct
observational way for you to tell; the virtual reality that you
would be living in would look and feel perfectly real. 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.
Now we get to the core of the simulation argument.
This does not purport to demonstrate
that you are in a simulation. Instead, it shows that we should
accept as true at least one of the following three propositions:
The chances that a species at
our current level of development can avoid going extinct
before becoming technologically mature is negligibly small
Almost no technologically mature
civilizations are interested in running computer simulations
of minds like ours
You are almost certainly in a
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).
While the full simulation argument employs some probability theory
and formalism, the gist of it can be understood in intuitive terms.
Suppose that proposition (1) is false. 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.
Therefore, if both (1) and (2) are false, there will be an
astronomically huge number of simulated minds like ours. If we work
out the numbers, we find that there would be vastly many more such
simulated minds than there would be non-simulated minds running on
organic brains. In other words, almost all minds like yours, having
the kinds of experiences that you have, would be simulated rather
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.
So if you think that (1) and (2) are both false, you should accept
(3). It is not coherent to reject all three propositions. In
reality, we do not have much specific information to tell us which
of the three propositions might be true. In this situation, it might
be reasonable to distribute our credence roughly evenly between the
three possibilities, giving each of them a substantial probability.
Let us consider the options in a little more detail. Possibility (1)
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.
Possibility (2) requires that there is a strong convergence among
all sufficiently advanced civilizations: almost none of them is
interested in running computer simulations of minds like ours, and
almost none of them contains any relatively wealthy individuals who
are interested in doing that and are free to act on their desires.
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.
The third possibility is the philosophically most intriguing. If (3)
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?
Your first reaction might think that if (3) is true, then all bets
are off, and that one would go crazy if one seriously thought that
one was living in a simulation.
To reason thus would be an error. Even if we were in a simulation,
the best way to predict what would happen next in our simulation is
still the ordinary methods – extrapolation of past trends,
scientific modeling, common sense and so on.
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.
The simulation hypothesis, however, may have some subtle effects on
rational everyday behavior.
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
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
Your fate in that afterlife could be
made to depend on how you behaved in your present simulated
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.
If we are in a simulation, is it possible that we could know that
for certain? If the simulators don’t want us to find out, we
probably never will. 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
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).