The Matrix as Metaphysics, David J. Chalmers

The Matrix as Metaphysics, David J. Chalmers
http://www.u.arizona.edu/~chalmers/papers/matrix.html
http://whatisthematrix.warnerbros.com/rl_cmp/new_phil_fr_chalmers.html
http://whatisthematrix.warnerbros.com/

------------------------------------

1. The Matrix presents a version of an old philosophical fable: the brain in
a vat. A disembodied brain is floating in a vat, inside a scientist's
laboratory. The scientist has arranged that the brain will be stimulated
with the same sort of inputs that a normal embodied brain receives. To do
this, the brain is connected to a giant computer simulation of a world. The
simulation determines which inputs the brain receives. When the brain
produces outputs, these are fed back into the simulation. The internal state
of the brain is just like that of a normal brain, despite the fact that it
lacks a body. From the brain's point of view, things seem very much as they
seem to you and me.

------------------------------------

2. Envatment Reconsidered; Even if I am in a matrix, my world is perfectly
real. A brain in a vat is not massively deluded (at least if it has always
been in the vat). Neo does not have massively false beliefs about the
external world. Instead, envatted beings have largely correct beliefs about
their world. If so, the Matrix Hypothesis is not a skeptical hypothesis, and
its possibility does not undercut everything that I think I know.

------------------------------------

3 The Metaphysical Hypothesis; The hypothesis that I am envatted is not a
skeptical hypothesis, but a metaphysical hypothesis. That is, it is a
hypothesis about the underlying nature of reality. Where physics is
concerned with the microscopic processes that underlie macroscopic reality,
metaphysics is concerned with the fundamental nature of reality. A
metaphysical hypothesis might make a claim about the reality that underlies
physics itself. Alternatively, it might say something about the nature of
our minds, or the creation of our world.

   (1) The Creation Hypothesis:
    Physical space-time and its
    contents were created by beings
    outside physical space-time.

   (2) The Computational Hypothesis:
    Microphysical processes throughout
    space-time are constituted by
    underlying computational processes.

   (3) The Mind-Body Hypothesis;
    My mind is (and has always been)
    constituted by processes outside
    physical space-time, and receives
    its perceptual inputs from and sends
    its outputs to processes in
    physical space-time.

   (4) The Metaphysical Hypothesis;
    The Combination Hypothesis, which
    combines all three [prior points].
    It says that physical space-time
    and its contents were created by
    beings outside physical space-time,
    that microphysical processes are
    constituted by computational processes,
    and that our minds are outside physical
    space-time but interact with it.

------------------------------------

4. The Matrix Hypothesis as a Metaphysical Hypothesis; Recall that the
Matrix Hypothesis says: I have (and have always had) a cognitive system that
receives its inputs from and sends its outputs to an artificially-designed
computer simulation of a world. The Matrix Hypothesis is equivalent to the
Metaphysical Hypothesis, in the following sense: if I accept the
Metaphysical Hypothesis, I should accept the Matrix Hypothesis, and if I
accept the Matrix Hypothesis, I should accept the Metaphysical Hypothesis.
That is, the two hypotheses imply each other, where this means that if one
accepts the one, one should accept the other.

------------------------------------

5. Life in the Matrix; The Matrix Hypothesis is not a skeptical hypothesis.
If I accept it, I should not infer that the external world does not exist,
or that I have no body, or that there are no tables and chairs, or that I am
not in Tucson. Rather, I should infer that the physical world is constituted
by computations beneath the microphysical level. There are still tables,
chairs, and bodies: these are made up fundamentally of bits, and of whatever
constitutes these bits. This world was created by other beings, but is still
perfectly real. My mind is separate from physical processes, and interacts
with them. My mind may not have been created by these beings, and it may not
be made up of bits, but it still interacts with these bits.

------------------------------------

6. Objection: Simulation is not Reality; A common line of objection is that
a simulation is not the same as reality. The Matrix Hypothesis implies only
that a simulation of physical processes exists. By contrast, the
Metaphysical Hypothesis implies that physical processes really exist (they
are explicitly mentioned in the Computational Hypothesis and elsewhere). If
so, then the Matrix Hypothesis cannot imply the Metaphysical Hypothesis. On
this view, if I am in a matrix, then physical processes do not really exist.
It is clearly possible that a computational level underlies real physical
processes, and it is possible that the computations here are implemented by
further processes in turn.

------------------------------------
7. Other Objections;

- Objection 1: A brain in a vat may think it is outside walking in the sun,
when in fact it is alone in a dark room. Surely it is deluded! Response: The
brain is alone in a dark room. But this does not imply that the person is
alone in a dark room.

- Objection 2: An envatted being may believe that it is in Tucson, when in
fact it is in New York, and has never been anywhere near Tucson. Surely this
belief is deluded. Response: The envatted being's concept of "Tucson" does
not refer to what we call Tucson. Rather, it refers to something else
entirely: call this Tucson*, or "virtual Tucson". We might think of this as
a "virtual location" (more on this in a moment). When the being says to
itself "I am in Tucson", it really is thinking that it is in Tucson*, and it
may well in fact be in Tucson*. Because Tucson is not Tucson*, the fact that
the being has never been in Tucson is irrelevant to whether its belief is
true.

- Objection 3: Before he leaves the Matrix, Neo believes that he has hair.
But in reality he has no hair (the body in the vat is bald). Surely this
belief is deluded. Response: This case is like the last one. Neo's concept
of "hair" does not refer to real hair, but to something else that we might
call hair* ("virtual hair"). So the fact that Neo does not have real hair is
irrelevant to whether his belief is true. Neo really does has virtual hair,
so he is correct. Likewise, when a child in the movie tells Neo "There is no
spoon", his concept refers to a virtual spoon, and there really is a virtual
spoon. So the child is wrong.

- Objection 4: What sort of objects does an envatted being refer to. What is
virtual hair, virtual Tucson, and so on? Response: These are all entities
constituted by computational processes. If I am envatted, then the objects
that I refer to (hair, Tucson, and so on) are all made of bits. And if
another being is envatted, the objects that it refers to (hair*, Tucson*,
and so on) are likewise made of bits. If the envatted being is hooked up to
a simulation in my computer, then the objects it refers to are constituted
by patterns of bits inside my computer. We might call these things virtual
objects. Virtual hands are not hands (assuming I am not envatted), but they
exist inside the computer all the same. Virtual Tucson is not Tucson, but it
exists inside the computer all the same.

- Objection 5: You just said that virtual hands are not real hands. Does
this mean that if we are in the matrix, we don't have real hands? Response:
No. If we are not in the matrix, but someone else is, we should say that
their term "hand" refers to virtual hands, but our term does not. So in this
case, our hands aren't virtual hands. But if we are in the matrix, then our
term "hand" refers to something that's made of bits: virtual hands, or at
least something that would be regarded as virtual hands by people in the
next world up. That is, if we are in the matrix, real hands are made of
bits. Things look quite different, and our words refer to different things,
depending on whether our perspective is inside or outside the matrix.

- Objection 6: Just which pattern of bits is a given virtual object? Surely
it will be impossible to pick out a precise set. Response: This question is
like asking: just which part of the quantum wavefunction is this chair, or
is the University of Arizona? These objects are all ultimately constituted
by an underlying quantum wavefunction, but there may be no precise part of
the micro-level wavefunction that we can say "is" the chair or the
university. The chair and the university exist at a higher level. Likewise,
if we are envatted, there may be no precise set of bits in the micro-level
computational process that is the chair or the university. These exist at a
higher level. And if someone else is envatted, there may be no precise sets
of bits in the computer simulation that "are" the objects they refer to. But
just as a chair exists without being any precise part of the wavefunction, a
virtual chair may exist without being any precise set of bits.

- Objection 7: An envatted being thinks it performs actions, and it thinks
it has friends. Are these beliefs correct? Response: One might try to say
that the being performs actions* and that it has friends*. But for various
reason I think it is not plausible that words like "action" and "friend" can
shift their meanings as easily as words like like "Tucson" and "hair".
Instead, I think one can say truthfully (in our own language) that the
envatted being performs actions, and that it has friends. To be sure, it
performs actions in its environment, and its environment is not our
environment but the virtual environment. And its friends likewise inhabit
the virtual environment (assuming that we have a multi-vat matrix, or that
computation suffices for consciousness). But the envatted being is not
incorrect in this respect.

- Objection 8: Set these technical points aside. Surely, if we are in a
matrix, the world is nothing like we think it is! Response: I deny this.
Even if we are in a matrix, there are still people, football games, and
particles, arranged in space-time just as we think they are. It is just that
the world has a further nature that goes beyond our initial conception. In
particular, things in the world are realized computationally in a way that
we might not have originally imagined. But this does not contradict any of
our ordinary beliefs. At most, it will contradict a few of our more abstract
metaphysical beliefs. But exactly the same goes for quantum mechanics,
relativity theory, and so on. If we are in a matrix, we may not have many
false beliefs, but there is much knowledge that we lack. For example, we do
not know that the universe is realized computationally. But this is just
what one should expect. Even if we are not in a matrix, there may well be
much about the fundamental nature of reality that we do not know. We are not
omniscient creatures, and our knowledge of the world is at best partial.
This is simply the condition of a creature living in a world.

------------------------------------

8. Other Skeptical Hypothesis

- New Matrix Hypothesis: I was recently created, along with all my memories,
and was put in a newly-created matrix.

- Recent Matrix Hypothesis: For most of my life I have not been envatted,
but I was recently hooked up to a matrix.

- Local Matrix Hypothesis: I am hooked up to a computer simulation of a
fixed local environment in a world.

- Extendible Local Matrix Hypothesis: I am hooked up to a computer
simulation of a local environment in a world, extended when necessary
depending on subject's movements.

- Macroscopic Matrix Hypothesis: I am hooked up to a computer simulation of
macroscopic physical processes without microphysical detail.

- God Hypothesis: Physical reality is represented in the mind of God, and
our own thoughts and perceptions depend on God's mind.

- Evil Genius Hypothesis: I have a disembodied mind, and an evil genius is
feeding me sensory inputs to give the appearance of an external world.

- Dream Hypothesis: I am now and have always been dreaming.

- Chaos Hypothesis: I do not receive inputs from anywhere in the world.
Instead, I have random uncaused experiences. Through a huge coincidence,
they are exactly the sort of regular, structured experiences with which I am
familiar.

------------------------------------

9 Philosophical Notes

Note 1: Hilary Putnam (1981) has argued that the hypothesis that I am (and
have always been) a brain in a vat can be ruled out a priori.

Note 2: A brain in a vat may have true beliefs, because it will refer to
chemical processes or processes inside a computer.

Note 3: A skeptical hypothesis with respect to a class of beliefs is one
that is a skeptical hypothesis with respect to most or all the beliefs in
that class. A global skeptical hypothesis is a skeptical hypothesis with
respect to all our empirical beliefs.

Note 4: What is the relevant class of beliefs? Of course there are some
beliefs that even a no-external-world skeptical hypothesis might not
undercut: the belief that I exist, or the belief that 2+2=4, or the belief
that there are no unicorns.

Note 5: On the Computational Hypothesis: It is coherent to suppose that
there is a computational level underneath physics, but it is not clear
whether it is coherent to suppose that this level is fundamental.

Note 6: On the Mind-Body Hypothesis: It is interesting to note that the
Matrix Hypothesis shows a concrete way in which Cartesian substance dualism
might have turned out to be true.

Note 7: That the Matrix Hypothesis "implies" the Metaphysical Hypothesis and
vice versa, indicates an epistemic relation: if one accepts the first, one
should accept the second.

Note 8: A causal account of reference. I said the truth of an envatted
being's thoughts depends not on its immediate environment but on what it is
causally connected to: that is, on the computational processes to which it
is hooked up. As noted earlier, I did not need to assume the causal theory
of reference to get to this conclusion, but instead got there through a
first-order argument.

Note 9: While an envatted beings' term "hand" or "hair" or "Tucson" may mean
something different from our corresponding term, an envatted beings' term
"friend" or "philosopher" or "action" will arguably mean the same as ours.

Note 10: What is the ontology of virtual objects? We should reject claims of
token identity between microscopic and macroscopic levels. Tables are not
identical to any object characterized purely in terms of quantum-mechanics;
likewise, virtual tables are not identical to any objects characterized
purely in terms of bits. But nevertheless, facts about tables supervene on
quantum-mechanical facts, and facts about virtual tables supervene on
computational facts. So it seems reasonable to say that tables are
constituted by quantum processes, and that virtual tables are constituted by
computational processes.

Note 11: We do not know the intrinsic nature of entities in the external
world. When it comes to physical entities, perception and science may tell
us how these entities affect us, and how they relate to each other, but
these methods tell us little about what the fundamental physical entities
are like in themselves. That is, these methods reveal the causal structure
of the external world, but they leave its intrinsic nature open.

Note 12: One general moral is that the "manifest image" is robust: our
ordinary conception of the macroscopic world is not easily falsified by
discoveries in science and metaphysics. As long as the physical world
contains processes with the right sort of causal and counterfactual
structure, then it will be compatible with the manifest image. Even a
computer simulation has the relevant causal and counterfactual structure, as
does a process in the mind of God: this is why they can support a robust
external reality, despite their surprising nature.

Note 13: To implement a formal computation it is required that the
implementation have concrete states that map directly onto these formal
states, where the pattern of (causal and counterfactual) interaction between
these states precisely mirrors the pattern of interaction between the formal
states. Any two implementations of the computation will share a certain
specific causal structure. A computational description of the physical world
will be required to mirror its causal structure down to the level of
fundmental objects and properties. So any implementation of this computation
will embody this causal structure (in transitions between implementing
states, whether these be voltages, circuits, or something quite different).
So insofar as our conception of the external world imposes constraints on
causal structure that a real physical world can satisfy, these constraints
will also be satisfied by a computer simulation.

Note 14: There are further constraints that our beliefs impose on the world
that the Matrix Hypothesis does not satisfy. One could argue that a mere
match in mental and causal structure is not enough. For example, one might
argue that the world needs to have the right spatial properties, where we
have some sort of direct grip on what spatial properties are (perhaps
because spatial concepts are semantically neutral). And one could suggest
that the problem with the matrix is that its spatial properties are all
wrong. We believe that external entities are arranged in a certain spatial
pattern, but no such spatial pattern exists inside the computer.

Note 15: The reasoning in this paper does not offer a knockdown refutation
of skepticism, as several skeptical hypotheses are left open. But I think it
significantly strengthens one of the standard responses to skepticism. It is
often held that although various skeptical hypotheses are compatible with
our experiences, the hypothesis that there is a real physical world provides
a simpler or better explanation of the regularities in our experiences than
these skeptical hypotheses. If so, then we may be justified in believing in
the real physical world, by an inference to the best explanation.

The Matrix as Metaphysics, David J. Chalmers
http://www.u.arizona.edu/~chalmers/papers/matrix.html
http://whatisthematrix.warnerbros.com/rl_cmp/new_phil_fr_chalmers.html
http://whatisthematrix.warnerbros.com/

===========================
END SUMMARY & BEGIN DOCUMENT
===========================

The Matrix as Metaphysics, David J. Chalmers
http://www.u.arizona.edu/~chalmers/papers/matrix.html

*[[This paper was written for the "philosophy section" of the official
Matrix website. As such, the bulk of the paper is written to be accessible
for an audience without a background in philosophy. At the same time, this
paper is intended as a serious work of philosophy, with relevance for
central issues in epistemology, metaphysics, and the philosophy of mind and
language. A section of "philosophical notes" at the end of the article draws
out some of these connections explicitly.]]

---------------------------------

1 Brains in Vats

The Matrix presents a version of an old philosophical fable: the brain in a
vat. A disembodied brain is floating in a vat, inside a scientist's
laboratory. The scientist has arranged that the brain will be stimulated
with the same sort of inputs that a normal embodied brain receives. To do
this, the brain is connected to a giant computer simulation of a world. The
simulation determines which inputs the brain receives. When the brain
produces outputs, these are fed back into the simulation. The internal state
of the brain is just like that of a normal brain, despite the fact that it
lacks a body. From the brain's point of view, things seem very much as they
seem to you and me.

The brain is massively deluded, it seems. It has all sorts of false beliefs
about the world. It believes that it has a body, but it has no body. It
believes that it is walking outside in the sunlight, but in fact it is
inside a dark lab. It believes it is one place, when in fact it may be
somewhere quite different. Perhaps it thinks it is in Tucson, when it is
actually in Australia, or even in outer space.

Neo's situation at the beginning of The Matrix is something like this. He
thinks that he lives in a city, he thinks that he has hair, he thinks it is
1999, and he thinks that it is sunny outside. In reality, he is floating in
space, he has no hair, the year is around 2199, and the world has been
darkened by war. There are a few small differences from the vat scenario
above: Neo's brain is located in a body, and the computer simulation is
controlled by machines rather than by a scientist. But the essential details
are much the same. In effect, Neo is a brain in a vat.

Let's say that a matrix (lower-case "m") is an artificially-designed
computer simulation of a world. So the Matrix in the movie is one example of
a matrix. And let's say that someone is envatted, or that they are in a
matrix, if they have a cognitive system which receives its inputs from and
sends its outputs to a matrix. Then the brain at the beginning is envatted,
and so is Neo.

We can imagine that a matrix simulates the entire physics of a world,
keeping track of every last particle throughout space and time. (Later, we
will look at ways in which this set-up might be varied.) An envatted being
will be associated with a particular simulated body. A connection is
arranged so that whenever this body receives sensory inputs inside the
simulation, the envatted cognitive system will receive sensory inputs of the
same sort. When the envatted cognitive system produces motor outputs,
corresponding outputs will be fed to the motor organs of the simulated body.

When the possibility of a matrix is raised, a question immediately follows.
How do I know that I am not in a matrix? After all, there could be a brain
in a vat structured exactly like my brain, hooked up to a matrix, with
experiences indistinguishable from those I am having now. From the inside,
there is no way to tell for sure that I am not in the situation of the brain
in a vat. So it seems that there is no way to know for sure that I am not in
a matrix.

Let us call the hypothesis that I am in a matrix and have always been in a
matrix the Matrix Hypothesis. Equivalently, the Matrix Hypothesis says that
I am envatted and have always been envatted. This is not quite equivalent to
the hypothesis that I am in the Matrix, as the Matrix is just one specific
version of a matrix. For now, I will ignore the some complications that are
specific to the Matrix in the movie, such as the fact that people sometimes
travel back and forth between the Matrix and the external world. These
issues aside, we can think of the Matrix Hypothesis informally as saying
that I am in the same sort of situation as people who have always been in
the Matrix.

The Matrix Hypothesis is one that we should take seriously. As Nick Bostrom
has suggested, it is not out of the question that in the history of the
universe, technology will evolve that will allow beings to create computer
simulations of entire worlds. There may well be vast numbers of such
computer simulations, compared to just one real world. If so, there may well
be many more beings who are in a matrix than beings who are not. Given all
this, one might even infer that it is more likely that we are in a matrix
than that we are not. Whether this is right or not, it certainly seems that
we cannot be certain that we are not in a matrix.

Serious consequences seem to follow. My envatted counterpart seems to be
massively deluded. It thinks it is in Tucson; it thinks it is sitting at a
desk writing an article; it thinks it has a body. But on the face of it, all
of these beliefs are false. Likewise, it seems that if I am envatted, my own
corresponding beliefs are false. If I am envatted, I am not really in
Tucson, I am not really sitting at a desk, and I may not even have a body.
So if I don't know that I am not envatted, then I don't know that I am in
Tucson, I don't know that I am sitting at a desk, and I don't know that I
have a body.

The Matrix Hypothesis threatens to undercut almost everything I know. It
seems to be a skeptical hypothesis: a hypothesis that I cannot rule out, and
one that would falsify most of my beliefs if it were true. Where there is a
skeptical hypothesis, it looks like none of these beliefs count as genuine
knowledge. Of course the beliefs might be true — I might be lucky, and not
be envatted — but I can't rule out the possibility that they are false. So a
skeptical hypothesis leads to skepticism about these beliefs: I believe
these things, but I do not know them.

To sum up the reasoning: I don't know that I'm not in a matrix. If I'm in a
matrix, I'm probably not in Tucson. So if I don't know that I'm not in a
matrix, then I don't know that I'm in Tucson. The same goes for almost
everything else I think I know about the external world.

-----------------------------

2 Envatment Reconsidered

This is a standard way of thinking about the vat scenario. It seems that
this view is also endorsed by the people who created The Matrix. On the DVD
case for the movie, one sees the following:

Perception: Our day-in, day-out world is real.

Reality: That world is a hoax, an elaborate deception spun by all-powerful
machines that control us. Whoa.

I think this view is not quite right. I think that even if I am in a matrix,
my world is perfectly real. A brain in a vat is not massively deluded (at
least if it has always been in the vat). Neo does not have massively false
beliefs about the external world. Instead, envatted beings have largely
correct beliefs about their world. If so, the Matrix Hypothesis is not a
skeptical hypothesis, and its possibility does not undercut everything that
I think I know.

Philosophers have held this sort of view before. The 18th-century
philosopher George Berkeley held, in effect, that appearance is reality.
(Recall Morpheus: "What is real? How do you define real? If you're talking
about what you can feel, what you can smell, what you can taste and see,
then real is simply electrical signals interpreted by your brain.") If this
is right, then the world perceived by envatted beings is perfectly real:
they have all the right appearances, and appearance is reality. So on this
view, even envatted beings have true beliefs about the world.

I have recently found myself embracing a similar conclusion, though for
quite different reasons. I don't find the view that appearance is reality
plausible, so I don't endorse Berkeley's reasoning. And until recently, it
has seemed quite obvious to me that brains in vats would have massively
false beliefs. But I now think there is a line of reasoning that shows that
this is wrong.

I still think I cannot rule out the hypothesis that I am in a matrix. But I
think that even I am in a matrix, I am still in Tucson, I am still sitting
at my desk, and so on. So the hypothesis that I am in a matrix is not a
skeptical hypothesis. The same goes for Neo. At the beginning of the film,
if he thinks "I have hair", he is correct. If he thinks "It is sunny
outside", he is correct. And the same goes, of course, for the original
brain in a vat. When it thinks "I have a body", it is correct. When it
thinks "I am walking", it is correct.

This view may seem very counterintuitive at first. Initially, it seemed
quite counterintuitive to me. So I'll now present the line of reasoning that
has convinced me that it is correct.

-----------------------------

3 The Metaphysical Hypothesis

I will argue that the hypothesis that I am envatted is not a skeptical
hypothesis, but a metaphysical hypothesis. That is, it is a hypothesis about
the underlying nature of reality.

Where physics is concerned with the microscopic processes that underlie
macroscopic reality, metaphysics is concerned with the fundamental nature of
reality. A metaphysical hypothesis might make a claim about the reality that
underlies physics itself. Alternatively, it might say something about the
nature of our minds, or the creation of our world.

I think the Matrix Hypothesis should be regarded as a metaphysical
hypothesis with all three of these elements. It makes a claim about the
reality underlying physics, about the nature of our minds, and about the
creation of the world.

In particular, I think the Matrix Hypothesis is equivalent to a version of
the following three-part Metaphysical Hypothesis. First, physical processes
are fundamentally computational. Second, our cognitive systems are separate
from physical processes, but interact with these processes. Third, physical
reality was created by beings outside physical space-time.

Importantly, nothing about this Metaphysical Hypothesis is skeptical. The
Metaphysical Hypothesis here tells us about the processes underlying our
ordinary reality, but it does not entail that this reality does not exist.
We still have bodies, and there are still chairs and tables: it's just that
their fundamental nature is a bit different from what we may have thought.
In this manner, the Metaphysical Hypothesis is analogous to a physical
hypotheses, such as one involving quantum mechanics. Both the physical
hypothesis and the Metaphysical Hypothesis tells us about the processes
underlying chairs. They do not entail that there are no chairs. Rather, they
tell us what chairs are really like.

I will make the case by introducing each of the three parts of the
Metaphysical Hypothesis separately. I will suggest that each of them is
coherent, and cannot be conclusively ruled out. And I will suggest that none
of them is a skeptical hypothesis: even if they are true, most of our
ordinary beliefs are still correct. The same goes for a combination of all
three hypothesis. I will then argue that the Matrix Hypothesis hypothesis is
equivalent to this combination.

(1) The Creation Hypothesis

The Creation Hypothesis says: Physical space-time and its contents were
created by beings outside physical space-time.

This is a familiar hypothesis. A version of it is believed by many people in
our society, and perhaps by the majority of the people in the world. If one
believes that God created the world, and if one believes that God is outside
physical space-time, then one believes the Creation Hypothesis. One needn't
believe in God to believe the Creation Hypothesis, though. Perhaps our world
was created by a relatively ordinary being in the "next universe up", using
the latest world-making technology in that universe. If so, the Creation
Hypothesis is true.

I don't know whether the Creation Hypothesis is true. But I don't know for
certain that it is false. The hypothesis is clearly coherent, and I cannot
conclusively rule it out.

The Creation Hypothesis is not a skeptical hypothesis. Even if it is true,
most of my ordinary beliefs are still true. I still have hands, I am still
in Tucson, and so on. Perhaps a few of my beliefs will turn out false: if I
am an atheist, for example, or if I believe all reality started with the Big
Bang. But most of my everyday beliefs about the external world will remain
intact.

(2) The Computational Hypothesis

The Computational Hypothesis says: Microphysical processes throughout
space-time are constituted by underlying computational processes.

The Computational Hypothesis says that physics as we know it not the
fundamental level of reality. Just as chemical processes underlie biological
processes, and microphysical processes underlie chemical processes,
something underlies microphysical processes. Underneath the level of quarks
and electrons and photons is a further level: the level of bits. These bits
are governed by a computational algorithm, which at a higher-level produces
the processes that we think of as fundamental particles, forces, and so on.

The Computational Hypothesis is not as widely believed as the Creation
Hypothesis, but some people take it seriously. Most famously, Ed Fredkin has
postulated that the universe is at bottom some sort of computer. More
recently, Stephen Wolfram has taken up the idea in his book A New Kind of
Science, suggesting that at the fundamental level, physical reality may be a
sort of cellular automata, with interacting bits governed by simple rules.
And some physicists have looked into the possibility that the laws of
physics might be formulated computationally, or could be seen as the
consequence of certain computational principles.

One might worry that pure bits could not be the fundamental level of
reality: a bit is just a 0 or a 1, and reality can't really be zeroes and
ones. Or perhaps a bit is just a "pure difference" between two basic states,
and there can't be a reality made up of pure differences. Rather, bits
always have to be implemented by more basic states, such as voltages in a
normal computer.

I don't know whether this objection is right. I don't think it's completely
out of the question that there could be a universe of "pure bits". But this
doesn't matter for present purposes. We can suppose that the computational
level is itself constituted by an even more fundamental level, at which the
computational processes are implemented. It doesn't matter for present
purposes what that more fundamental level is. All that matters is that
microphysical processes are constituted by computational processes, which
are themselves constituted by more basic processes. From now on I will
regard the Computational Hypothesis as saying this.

I don't know whether the Computational Hypothesis is correct. But again, I
don't know that it is false. The hypothesis is coherent, if speculative, and
I cannot conclusively rule it out.

The Computational Hypothesis is not a skeptical hypothesis. If it is true,
there are still electrons and protons. On this picture, electrons and
protons will be analogous to molecules: they are made up of something more
basic, but they still exist. Similarly, if the Computational Hypothesis is
true, there are still tables and chairs, and macroscopic reality still
exists. It just turns out that their fundamental reality is a little
different from what we thought.

The situation here is analogous to that with quantum mechanics or
relativity. These may lead us to revise a few "metaphysical" beliefs about
the external world: that the world is made of classical particles, or that
there is absolute time. But most of our ordinary beliefs are left intact.
Likewise, accepting the Computational Hypothesis may lead us to revise a few
metaphysical beliefs: that electrons and protons are fundamental, for
example. But most of our ordinary beliefs are unaffected.

(3) The Mind-Body Hypothesis

The Mind-Body Hypothesis says: My mind is (and has always been) constituted
by processes outside physical space-time, and receives its perceptual inputs
from and sends its outputs to processes in physical space-time.

The Mind-Body Hypothesis is also quite familiar, and quite widely believed.
Descartes believed something like this: on his view, we have nonphysical
minds that interact with our physical bodies. The hypothesis is less widely
believed today than in Descartes' time, but there are still many people who
accept the Mind-Body Hypothesis.

Whether or not the Mind-Body Hypothesis is true, it is certainly coherent.
Even if contemporary science tends to suggest that the hypothesis is false,
we cannot rule it out conclusively.

The Mind-Body Hypothesis is not a skeptical hypothesis. Even if my mind is
outside physical space-time, I still have a body, I am still in Tucson, and
so on. At most, accepting this hypothesis would make us revise a few
metaphysical belies about our minds. Our ordinary beliefs about external
reality will remain largely intact.

(4) The Metaphysical Hypothesis

We can now put these hypotheses together. First we can consider the
Combination Hypothesis, which combines all three. It says that physical
space-time and its contents were created by beings outside physical
space-time, that microphysical processes are constituted by computational
processes, and that our minds are outside physical space-time but interact
with it.

As with the hypotheses taken individually, the Combination Hypothesis is
coherent, and we cannot conclusively rule it out. And like the hypotheses
taken individually, it is not a skeptical hypothesis. Accepting it might
lead us to revise a few of our beliefs, but it would leave most of them
intact.

Finally, we can consider the Metaphysical Hypothesis (with a capital M).
Like the Combination Hypothesis, this combines the Creation Hypothesis, the
Computational Hypothesis, and the Mind-Body Hypothesis. It also adds the
following more specific claim: the computational processes underlying
physical space-time were designed by the creators as a computer simulation
of a world.

(It may also be useful to think of the Metaphysical Hypothesis as saying
that the computational processes constituting physical space-time are part
of a broader domain, and that the creators and my cognitive system are also
located within this domain. This addition is not strictly necessary for what
follows, but it matches up with the most common way of thinking about the
Matrix Hypothesis.)

The Metaphysical Hypothesis is a slightly more specific version of the
Combination Hypothesis, in that in specifies some relations between the
various parts of the hypothesis. Again, the Metaphysical Hypothesis is a
coherent hypothesis, and we cannot conclusively rule it out. And again, it
is not a skeptical hypothesis. Even if we accept it, most of our ordinary
beliefs about the external world will be left intact.

--------------------------

4 The Matrix Hypothesis as a Metaphysical Hypothesis

Recall that the Matrix Hypothesis says: I have (and have always had) a
cognitive system that receives its inputs from and sends its outputs to an
artificially-designed computer simulation of a world.

I will argue that the Matrix Hypothesis is equivalent to the Metaphysical
Hypothesis, in the following sense: if I accept the Metaphysical Hypothesis,
I should accept the Matrix Hypothesis, and if I accept the Matrix
Hypothesis, I should accept the Metaphysical Hypothesis. That is, the two
hypotheses imply each other, where this means that if one accepts the one,
one should accept the other.

Take the first direction first, from the Metaphysical Hypothesis to the
Matrix Hypothesis. The Mind-Body Hypothesis implies that I have (and have
always had) an isolated cognitive system which receives its inputs from and
sends its outputs to processes in physical space-time. In conjunction with
the Computational Hypothesis, this implies that my cognitive system receives
inputs from and sends outputs to the computational processes that constitute
physical space-time. The Creation Hypothesis (along with the rest of the
Metaphysical Hypothesis) implies that these processes were artificially
designed to simulate a world. It follows that I have (and have always had)
an isolated cognitive system that receives its inputs from and sends its
outputs to an artificially-designed computer simulation of a world. This is
just the Matrix Hypothesis. So the Metaphysical Hypothesis implies the
Matrix Hypothesis.

The other direction is closely related. To put it informally: If I accept
the Matrix Hypothesis, I accept that what underlies apparent reality is just
as the Metaphysical Hypothesis specifies. There is a domain containing my
cognitive system, causally interacting with a computer simulation of
physical-space time, which was created by other beings in that domain. This
just what has to obtain in order for the Metaphysical Hypothesis to obtain.
If one accepts this, one should accept the Creation Hypothesis, the
Computational Hypothesis, the Mind-Body Hypothesis, and the relevant
relations among these.

This may be a little clearer through a picture. Here is the shape of the
world according to the Matrix Hypothesis.

At the fundamental level, this picture of the shape of the world is exactly
the same as the picture of the Metaphysical Hypothesis given above. So if
one accepts that the world is as it is according to the Matrix Hypothesis,
one should accept that it is as it is according to the Metaphysical
Hypothesis.

One might make various objections. For example, one might object that the
Matrix Hypothesis implies that a computer simulation of physical processes
exists, but (unlike the Metaphysical Hypothesis) it does not imply that the
physical processes themselves exist. I will discuss this objection in
section 6, and other objections in section 7. For now, though, I take it
that there is a strong case that the Matrix Hypothesis implies the
Metaphysical Hypothesis, and vice versa.

--------------------------

5 Life in the Matrix

If this is right, it follows that the Matrix Hypothesis is not a skeptical
hypothesis. If I accept it, I should not infer that the external world does
not exist, or that I have no body, or that there are no tables and chairs,
or that I am not in Tucson. Rather, I should infer that the physical world
is constituted by computations beneath the microphysical level. There are
still tables, chairs, and bodies: these are made up fundamentally of bits,
and of whatever constitutes these bits. This world was created by other
beings, but is still perfectly real. My mind is separate from physical
processes, and interacts with them. My mind may not have been created by
these beings, and it may not be made up of bits, but it still interacts with
these bits.

The result is a complex picture of the fundamental nature of reality. The
picture is strange and surprising, perhaps, but it is a picture of a
full-blooded external world. If we are in a matrix, this is simply the way
that the world is.

We can think of the Matrix Hypothesis as a creation myth for the information
age. If it is correct, then the physical world was created, not necessarily
by gods. Underlying the physical world is a giant computation, and creators
created this world by implementing this computation. And our minds lie
outside this physical structure, with an independent nature that interacts
with this structure.

Many of the same issues that arise with standard creation myths arise here.
When was the world created? Strictly speaking, it was not created within our
time at all. When did history begin? The creators might have started the
simulation in 4004 BC (or in 1999) with the fossil record intact, but it
would have been much easier for them to start the simulation at the Big Bang
and let things run their course from there.

(In the movie Matrix, of course, the creators are machines. This gives an
interesting twist on common theological readings of the movie. It is often
held that Neo is the Christ figure in the movie, with Morpheus corresponding
to John the Baptist, Cypher to Judas Iscariot, and so on. But on the reading
I have given, the gods of the Matrix are the machines. Who, then, is the
Christ figure? Agent Smith, of course! After all, he is the gods' offspring,
sent down to save the Matrix world from those who wish to destroy it. And in
the second movie, he is even resurrected.)

Many of the same issues that arise on the standard Mind-Body Hypothesis also
arise here. When do our nonphysical minds start to exist? It depends on just
when new envatted cognitive systems are attached to the simulation (perhaps
at the time of conception within the matrix, or perhaps at time of birth?).
Is there life after death? It depends on just what happens to the envatted
systems once their simulated bodies die. How do mind and body interact? By
causal links that are outside physical space and time.

Even if we not in a matrix, we can extend a version of this reasoning to
other beings who are in a matrix. If they discover their situation, and come
to accept that they are in a matrix, they should not reject their ordinary
beliefs about the external world. At most, they should come to revise their
beliefs about the underlying nature of their world: they should come to
accept that external objects are made of bits, and so on. These beings are
not massively deluded: most of their ordinary beliefs about their world are
correct.

There are a few qualifications here. One may worry about beliefs about other
people's minds. I believe that my friends are conscious. If I am in a
matrix, is this correct? In the Matrix depicted in the movie, these beliefs
are mostly fine. This is a multi-vat matrix: for each of my perceived
friends, there is an envatted being in the external reality, who is
presumably conscious like me. The exception might be beings such as Agent
Smith, who is not envatted, but is entirely computational. Whether these
beings are conscious depends on whether computation is enough for
consciousness. I will remain neutral on that issue here. We could circumvent
this issue by building into the Matrix Hypothesis the requirement that all
the beings we perceive are envatted. But even if we do not build in this
requirement, we are not much worse off than in the actual world, where there
is a legitimate issue about whether other beings are conscious, quite
independently of whether we are in a matrix.

One might also worry about beliefs about the distant past, and about the far
future. These will be unthreatened as long as the computer simulation covers
all of space-time, from the Big Bang until the end of the universe. This is
built into the Metaphysical Hypothesis, and we can stipulate that it is
built into the Matrix Hypothesis too, by requiring that the computer
simulation be a simulation of an entire world. There may be other
simulations that start in the recent past (perhaps the Matrix in the movie
is like this), and there may be others that only last for a short while. In
these cases, the envatted beings will have false beliefs about the past
and/or the future in their worlds. But as long as the simulation covers the
lifespan of these beings, it is plausible that they will have mostly correct
beliefs about the current state of their environment.

There may be some respects in which the beings in a matrix are deceived. It
may be that the creators of the matrix control and interfere with much of
what happens in the simulated world. (The Matrix in the movie may be like
this, though the extent of the creators' control is not quite clear.) If so,
then these beings may have much less control over what happens than they
think. But the same goes if there is an interfering god in a non-matrix
world. And the Matrix Hypothesis does not imply that the creators interfere
with the world, though it leaves the possibility open. At worst, the Matrix
Hypothesis is no more skeptical in this respect than the Creation Hypothesis
in a non-matrix world.

The inhabitants of a matrix may also be deceived in that reality is much
bigger than they think. They might think their physical universe is all
there is, when in fact there is much more in the world, including beings and
objects that they can never possibly see. But again, this sort of worry can
arise equally in a non-matrix world. For example, cosmologists seriously
entertain the hypothesis that our universe may stem from a black hole in the
"next universe up", and that in reality there may be a whole tree of
universes. If so, the world is also much bigger than we think, and there may
be beings and objects that we can never possibly see. But either way, the
world that we see is perfectly real.

Importantly, none of these sources of skepticism — about other minds, the
past and the future, about our control over the world, and about the extent
of the world — casts doubt on our belief in the reality of the world that we
perceive. None of them leads us to doubt the existence of external objects
such as tables and chairs, in the way that the vat hypothesis is supposed to
do. And none of these worries is especially tied to the matrix scenario. One
can raise doubts about whether other minds exist, whether the past and the
future exist, and whether we have control over our worlds quite
independently of whether we are in a matrix. If this is right, then the
Matrix Hypothesis does not raise the distinctive skeptical issues that it is
often taken to raise.

I suggested before that it is not out of the question that we really are in
a matrix. One might have thought that this is a worrying conclusion. But if
I am right, it is not nearly as worrying as one might have thought. Even if
we are in such a matrix, our world is no less real than we thought it was.
It just has a surprising fundamental nature.

----------------------------------

6 Objection: Simulation is not Reality
(This slightly technical section can be skipped without too much loss.)

A common line of objection is that a simulation is not the same as reality.
The Matrix Hypothesis implies only that a simulation of physical processes
exists. By contrast, the Metaphysical Hypothesis implies that physical
processes really exist (they are explicitly mentioned in the Computational
Hypothesis and elsewhere). If so, then the Matrix Hypothesis cannot imply
the Metaphysical Hypothesis. On this view, if I am in a matrix, then
physical processes do not really exist.

In response: My argument does not require the general assumption that
simulation is the same as reality. The argument works quite differently. But
the objection helps us to flesh out the informal argument that the Matrix
Hypothesis implies the Metaphysical Hypothesis.

Because the Computational Hypothesis is coherent, it is clearly possible
that a computational level underlies real physical processes, and it is
possible that the computations here are implemented by further processes in
turn. So there is some sort of computational system that could yield reality
here. But here, the objector will hold that not all computational systems
are created equal. To say that some computational systems will yield real
physical processes in this role is not to say that they all do. Perhaps some
of them are merely simulations. If so, then the Matrix Hypothesis may not
yield reality.

To rebut this objection, we can appeal to two principles. First principle:
any abstract computation that could be used to simulate physical space-time
is such that it could turn out to underlie real physical processes. Second
principle: given an abstract computation that could underlie physical
processes, the precise way in which it is implemented is irrelevant to
whether it does underlie physical processes. In particular, the fact that
the implementation was designed as a simulation is irrelevant. The
conclusion then follows directly.

On the first principle: let us think of abstract computations in purely
formal terms, abstracting away from their manner of implementation. For an
abstract computation to qualify as a simulation of physical reality, it must
have computational elements that correspond to every particle in reality
(likewise for fields, waves, or whatever is fundamental), dynamically
evolving in a way that corresponds to the particle's evolution. But then, it
is guaranteed that the computation will have a rich enough causal structure
that it could in principle underlie physics in our world. Any computation
will do, as long as it has enough detail to correspond to the fine details
of physical processes.

On the second principle: given an abstract computation that could underlie
physical reality, it does not matter how the computation is implemented. We
can imagine discovering that some computational level underlies the level of
atoms and electrons. Once we have discovered this, it is possible that this
computational level is implemented by more basic processes. There are many
hypotheses about what the underlying processes could be, but none of them is
especially privileged, and none of them would lead us to reject the
hypothesis that the computational level constitutes physical processes. That
is, the Computational Hypothesis is implementation-independent: as long as
we have the right sort of abstract computation, the manner of implementation
does not matter.

In particular, it is irrelevant whether or not these implementing processes
were artificially created, and it is irrelevant whether they were intended
as a simulation. What matters is the intrinsic nature of the processes, not
their origin. And what matters about this intrinsic nature is simply that
they are arranged in such a way to implement the right sort of computation.
If so, the fact that the implementation originated as a simulation is
irrelevant to whether it can constitute physical reality.

There is one further constraint on the implementing processes: they must be
connected to our experiences in the right sort of way. That is when we have
an experience of an object, the processes underlying the simulation of that
object must be causally connected in the right sort of way to our
experiences. If this is not the case, then there will be no reason to think
that these computational processes underlie the physical processes that we
perceive. If there is an isolated computer simulation to which nobody is
connected in this way, we should say that it is simply a simulation. But an
appropriate hook-up to our perceptual experiences is built into the Matrix
Hypothesis, on the most natural understanding of that hypothesis. So the
Matrix Hypothesis has no problems here.

Overall, then, we have seen that a computational processes could underlie
physical reality, that any abstract computation that qualifies as a
simulation of physical reality could play this role, and that any
implementation of this computation could constitute physical reality, as
long as it is hooked up to our experiences in the relevant way. The Matrix
Hypothesis guarantees that we have an abstract computation of the right
sort, and it guarantees that it is hooked up to our experiences in the
relevant way. So the Matrix Hypothesis implies that the Computational
Hypothesis is correct, and that the computer simulation constitutes genuine
physical processes.

--------------------------------

7 Other Objections

When we look at a brain in a vat from the outside, it is hard to avoid the
sense that it is deluded. This sense manifests itself in a number of related
objections. These are not direct objections to the argument above, but they
are objections to its conclusion.

Objection 1: A brain in a vat may think it is outside walking in the sun,
when in fact it is alone in a dark room. Surely it is deluded!

Response: The brain is alone in a dark room. But this does not imply that
the person is alone in a dark room. By analogy, just say Descartes is right
that we have disembodied minds outside space-time, made of ectoplasm. When I
think "I am outside in the sun", an angel might look at my ectoplasmic mind
and note that in fact it is not exposed to any sun at all. Does it follow
that my thought is incorrect? Presumably not: I can be outside in the sun,
even if my ectoplasmic mind is not. The angel would be wrong to infer that I
have an incorrect belief. Likewise, we should not infer that envatted being
has an incorrect belief. At least, it is no more deluded than a Cartesian
mind.

The moral is that the immediate surroundings of our minds may well be
irrelevant to the truth of most of our beliefs. What matters is the
processes that our minds are connected to, by perceptual inputs and motor
outputs. Once we recognize this, the objection falls away.

Objection 2: An envatted being may believe that it is in Tucson, when in
fact it is in New York, and has never been anywhere near Tucson. Surely this
belief is deluded.

Response: The envatted being's concept of "Tucson" does not refer to what we
call Tucson. Rather, it refers to something else entirely: call this
Tucson*, or "virtual Tucson". We might think of this as a "virtual location"
(more on this in a moment). When the being says to itself "I am in Tucson",
it really is thinking that it is in Tucson*, and it may well in fact be in
Tucson*. Because Tucson is not Tucson*, the fact that the being has never
been in Tucson is irrelevant to whether its belief is true.

A rough analogy: I look at my colleague Terry, and think "that's Terry".
Elsewhere in the world, a duplicate of me looks at a duplicate of Terry. It
thinks "that's Terry", but it is not looking at the real Terry. Is its
belief false? It seems not: my duplicate's "Terry" concept refers not to
Terry, but to his duplicate Terry*. My duplicate really is looking at
Terry*, so its belief is true. The same sort of thing is happening in the
case above.

Objection 3: Before he leaves the Matrix, Neo believes that he has hair. But
in reality he has no hair (the body in the vat is bald). Surely this belief
is deluded.

Response: This case is like the last one. Neo's concept of "hair" does not
refer to real hair, but to something else that we might call hair* ("virtual
hair"). So the fact that Neo does not have real hair is irrelevant to
whether his belief is true. Neo really does has virtual hair, so he is
correct. Likewise, when a child in the movie tells Neo "There is no spoon",
his concept refers to a virtual spoon, and there really is a virtual spoon.
So the child is wrong.

Objection 4: What sort of objects does an envatted being refer to. What is
virtual hair, virtual Tucson, and so on?

Response: These are all entities constituted by computational processes. If
I am envatted, then the objects that I refer to (hair, Tucson, and so on)
are all made of bits. And if another being is envatted, the objects that it
refers to (hair*, Tucson*, and so on) are likewise made of bits. If the
envatted being is hooked up to a simulation in my computer, then the objects
it refers to are constituted by patterns of bits inside my computer. We
might call these things virtual objects. Virtual hands are not hands
(assuming I am not envatted), but they exist inside the computer all the
same. Virtual Tucson is not Tucson, but it exists inside the computer all
the same.

Objection 5: You just said that virtual hands are not real hands. Does this
mean that if we are in the matrix, we don't have real hands?

Response: No. If we are not in the matrix, but someone else is, we should
say that their term "hand" refers to virtual hands, but our term does not.
So in this case, our hands aren't virtual hands. But if we are in the
matrix, then our term "hand" refers to something that's made of bits:
virtual hands, or at least something that would be regarded as virtual hands
by people in the next world up. That is, if we are in the matrix, real hands
are made of bits. Things look quite different, and our words refer to
different things, depending on whether our perspective is inside or outside
the matrix.

This sort of perspective shift is common in thinking about the matrix
scenario. From the first-person perspective, we suppose that we are in a
matrix. Here, real things in our world are made of bits, though the "next
world up" might not be made of bits. From the third-person perspective, we
suppose that someone else is in a matrix but we are not. Here, real things
in our world are not made of bits, but the "next world down" is made of
bits. On the first way of doing things, our words refer to computational
entities. On the second way of doing things, the envatted beings' words
refer to computational entities, but our words do not.

Objection 6: Just which pattern of bits is a given virtual object? Surely it
will be impossible to pick out a precise set.

Response: This question is like asking: just which part of the quantum
wavefunction is this chair, or is the University of Arizona? These objects
are all ultimately constituted by an underlying quantum wavefunction, but
there may be no precise part of the micro-level wavefunction that we can say
"is" the chair or the university. The chair and the university exist at a
higher level. Likewise, if we are envatted, there may be no precise set of
bits in the micro-level computational process that is the chair or the
university. These exist at a higher level. And if someone else is envatted,
there may be no precise sets of bits in the computer simulation that "are"
the objects they refer to. But just as a chair exists without being any
precise part of the wavefunction, a virtual chair may exist without being
any precise set of bits.

Objection 7: An envatted being thinks it performs actions, and it thinks it
has friends. Are these beliefs correct?

Response: One might try to say that the being performs actions* and that it
has friends*. But for various reason I think it is not plausible that words
like "action" and "friend" can shift their meanings as easily as words like
like "Tucson" and "hair". Instead, I think one can say truthfully (in our
own language) that the envatted being performs actions, and that it has
friends. To be sure, it performs actions in its environment, and its
environment is not our environment but the virtual environment. And its
friends likewise inhabit the virtual environment (assuming that we have a
multi-vat matrix, or that computation suffices for consciousness). But the
envatted being is not incorrect in this respect.

Objection 8: Set these technical points aside. Surely, if we are in a
matrix, the world is nothing like we think it is!

Response: I deny this. Even if we are in a matrix, there are still people,
football games, and particles, arranged in space-time just as we think they
are. It is just that the world has a further nature that goes beyond our
initial conception. In particular, things in the world are realized
computationally in a way that we might not have originally imagined. But
this does not contradict any of our ordinary beliefs. At most, it will
contradict a few of our more abstract metaphysical beliefs. But exactly the
same goes for quantum mechanics, relativity theory, and so on.

If we are in a matrix, we may not have many false beliefs, but there is much
knowledge that we lack. For example, we do not know that the universe is
realized computationally. But this is just what one should expect. Even if
we are not in a matrix, there may well be much about the fundamental nature
of reality that we do not know. We are not omniscient creatures, and our
knowledge of the world is at best partial. This is simply the condition of a
creature living in a world.

-------------------------------------

8 Other Skeptical Hypothesis

The Matrix Hypothesis is one example of a traditional "skeptical"
hypothesis, but it is not the only example. Other skeptical hypotheses are
not quite as straightforward as the Matrix Hypothesis. Still, I think that
for many of them, a similar line of reasoning applies. In particular, one
can argue that most of these are not global skeptical hypothesis: that is,
their truth would not undercut all of our empirical beliefs about the
physical world. At worst, most of them are partial skeptical hypotheses,
undercutting some of our empirical beliefs, but leaving many of these
beliefs intact.

- New Matrix Hypothesis: I was recently created, along with all my memories,
and was put in a newly-created matrix.

What if both the matrix and I have existed for only a short time? This
hypothesis is a computational version of Bertrand Russell's Recent Creation
Hypothesis: the physical world was created only recently (with fossil record
intact), and so was I (with memories intact). On that hypothesis, the
external world that I perceive really exists, and most of my beliefs about
its current states are plausibly true, but I have many false beliefs about
the past. I think the same should be said of the New Matrix Hypothesis. One
can argue, along the lines presented earlier, that the New Matrix Hypothesis
is equivalent to a combination of the Metaphysical Hypothesis with the
Recent Creation Hypothesis. This combination is not a global skeptical
hypothesis (though it is a partial skeptical hypothesis, where beliefs about
the past are concerned). So the same goes for the New Matrix Hypothesis.

- Recent Matrix Hypothesis: For most of my life I have not been envatted,
but I was recently hooked up to a matrix.

If I was recently put in a matrix without realizing it, it seems that many
of my beliefs about my current environment are false. Let's say that just
yesterday someone put me into a simulation, in which I fly to Las Vegas and
gamble at a casino. Then I may believe that I am in Las Vegas now, and that
I am in a casino, but these beliefs at false: I am really in a laboratory in
Tucson.

This result is quite different from the long-term matrix. The difference
lies in the fact that my conception of external reality is anchored to the
reality in which I have lived most of my life. If I have been envatted all
my life, my conception is anchored to the computationally constituted
reality. But if I was just envatted yesterday, my conception is anchored to
the external reality. So when I think that I am in Las Vegas, I am thinking
that I am in the external Las Vegas, and this thought is false.

Still, this does not undercut all of my beliefs about the external world. I
believe that I was born in Sydney, that there is water in the oceans, and so
on, and all of these beliefs are correct. It is only my recently acquired
beliefs, stemming from perception of the simulated environment, that will be
false. So this is only a partial skeptical hypothesis: its possibility casts
doubt on a subset of our empirical beliefs, but it does not cast doubt on
all of them.

Interestingly, the Recent Matrix and the New Matrix hypothesis give opposite
results, despite their similar nature: the Recent Matrix Hypothesis yields
true beliefs about the past but false beliefs about the present, while the
New Matrix Hypothesis yields false beliefs about the past and true beliefs
about the present. The differences are tied to the fact that in Recent
Matrix Hypothesis, I really have a past existence for my beliefs to be
about, and that past reality has played a role in anchoring the contents of
my thoughts that has no parallel under the New Matrix Hypothesis.

- Local Matrix Hypothesis: I am hooked up to a computer simulation of a
fixed local environment in a world.

On one way of doing this, a computer simulates a small fixed environment in
a world, and the subjects in the simulation encounter some sort of barrier
when they try to leave that area. For example, in the movie The Thirteenth
Floor, just California is simulated, and when the subject tries to drive to
Nevada, the road says "Closed for Repair" (with faint green electronic
mountains in the distance!). Of course this is not the best way to create a
matrix, as subjects are likely to discover the limits to their world.

This hypothesis is analogous to a Local Creation Hypothesis, on which
creators just created a local part of the physical world. Under this
hypothesis, we will have true beliefs about nearby matters, but false
beliefs about matters further from home. By the usual sort of reasoning, the
Local Matrix Hypothesis can be seen as a combination of the Metaphysical
Hypothesis with the Local Creation Hypothesis. So we should say the same
thing about this.

- Extendible Local Matrix Hypothesis: I am hooked up to a computer
simulation of a local environment in a world, extended when necessary
depending on subject's movements.

This hypothesis avoids the obvious difficulties with a fixed local matrix.
Here the creators simulate a local environment and extend it when necessary.
For example, they might right now be concentrating on simulating a room in
my house in Tucson. If I walk into another room, or fly to another city,
they will simulate those. Of course they need to make sure that when I go to
these places, they match my memories and beliefs reasonably well, with
allowance for evolution in the meantime. The same goes for when I encounter
familiar people, or people I have only heard about. Presumably the
simulators keep up a database of the information about the world that has
been settled so far, updating this information whenever necessary as time
goes along, and making up new details when they need them.

This sort of simulation is quite unlike simulation in an ordinary matrix. In
a matrix, the whole world is simulated at once. There are high start-up
costs, but once the simulation is up and running, it will take care of
itself. By contrast, the extendible local matrix involves "just-in-time"
simulation. This has much lower start-up costs, but it requires much more
work and creativity as the simulation evolves.

This hypothesis is analogous to an Extendible Local Creation Hypothesis
about ordinary reality, under which creators create just a local physical
environment, and extend it when necessary. Here, external reality exists and
many local beliefs are true, but again beliefs about matters further from
home are false. If we combine that hypothesis with the Metaphysical
Hypothesis, the result is the Extendible Local Matrix Hypothesis. So if we
are in an extendible local matrix, external reality still exists, but there
is not as much of it as we thought. Of course if I travel in the right
direction, more of it may come into existence!

The situation is reminiscent of The Truman Show. Truman lives in an
artificial environment made up of actors and props, which behave
appropriately when he is around, but which may be completely different when
he is absent. Truman has many true beliefs about his current environment:
there really are tables and chairs in front of him, and so on. But he is
deeply mistaken about things outside his current environment, and further
from home.

It is common to think that while The Truman Show poses a disturbing
skeptical scenario, The Matrix is much worse. But if I am right, things are
reversed. If I am in a matrix, then most of my beliefs about the external
world are true. If I am in something like The Truman Show, then a great
number of my beliefs are false. On reflection, it seems to me that this is
the right conclusion. If we were to discover that we were (and always had
been) in a matrix, this would be surprising, but we would quickly get used
to it. If we were to discover that we were (and always had been) in the
Truman Show, we might well go insane.

- Macroscopic Matrix Hypothesis: I am hooked up to a computer simulation of
macroscopic physical processes without microphysical detail.

One can imagine that for ease of simulation, the makers of a matrix might
not both to simulate low-level physics. Instead, they might just represent
macroscopic objects in the world and their properties: e.g. that there is a
table with such-and-such shape, position, and color, with a book on top of
it with certain properties, and so on. They will need to make some effort to
make sure that these objects behave in a physically reasonable way, and they
will have to make special provisions for handling microphysical
measurements, but one can imagine that at least a reasonable simulation
could be created this way.

I think this hypothesis is analogous to a Macroscopic World Hypothesis:
there are no microphysical processes, and instead macroscopic physical
objects exist as fundamental objects in the world, with properties of shape,
color, position, and so on. This is a coherent way our world could be, and
it is not a global skeptical hypothesis, though it may lead to false
scientific beliefs about lower levels of reality. The Macroscopic Matrix
Hypothesis can be seen as a combination of this hypothesis with a version of
the Metaphysical Hypothesis. As such, it is not a global skeptical
hypothesis either.

One can also combine the various hypothesis above in various ways, yielding
hypotheses such as a New Local Macroscopic Matrix Hypothesis. For the usual
reasons, all of these can be seen as analogs of corresponding hypotheses
about the physical world. So all of them are compatible with the existence
of physical reality, and none is a global skeptical hypothesis.

- God Hypothesis: Physical reality is represented in the mind of God, and
our own thoughts and perceptions depend on God's mind.

A hypothesis like this was put forward by George Berkeley as a view about
how our world might really be. Berkeley intended this as a sort of
metaphysical hypothesis about the nature of reality. Most other philosophers
have differed from Berkeley in regarding this as a sort of skeptical
hypothesis. If I am right, Berkeley is closer to the truth. The God
Hypothesis can be seen as a version of the Matrix Hypothesis, on which the
simulation of the world is implemented in the mind of God. If this is right,
we should say that physical processes really exist: it's just that at the
most fundamental level, they are constituted by processes in the mind of
God.

- Evil Genius Hypothesis: I have a disembodied mind, and an evil genius is
feeding me sensory inputs to give the appearance of an external world.

This is Rene Descartes's classical skeptical hypothesis. What should we say
about it? This depends on just how the evil genius works. If the evil genius
simulates an entire world in his head in order to determine what inputs I
should receive, then we have a version of the God Hypothesis. Here we should
say that physical reality exists and is constituted by processes within the
genius. If the evil genius is simulating only a small part of the physical
world, just enough to give me reasonably consistent inputs, then we have an
analog of the Local Matrix Hypothesis (in either its fixed or flexible
versions). Here we should say that just a local part of external reality
exists. If the evil genius is not bothering to simulate the microphysical
level, but just the macroscopic level, then we have an analog of the
Macroscopic Matrix Hypothesis. Here we should say that local external
macroscopic objects exist, but our beliefs about their microphysical nature
are incorrect.

The evil genius hypothesis is often taken to be a global skeptical
hypothesis. But if the reasoning above is right, this is incorrect. Even if
the Evil Genius Hypothesis is correct, some of the external reality that we
apparently perceive really exists, though we may have some false beliefs
about it, depending on details. It is just that this external reality has an
underlying nature that is quite different from what we may have thought.

- Dream Hypothesis: I am now and have always been dreaming.

Descartes raised the question: how do you know that you are not currently
dreaming? Morpheus raises a similar question:

Have you ever had a dream, Neo, that you were so sure was real. What if you
were unable to wake from that dream? How would you know the difference
between the dream world and the real world?

The hypothesis that I am currently dreaming is analogous to a version of the
Recent Matrix Hypothesis. I cannot rule it out conclusively, and if it is
correct, then many of my beliefs about my current environment are incorrect.
But presumably I still have many true beliefs about the external world,
anchored in the past.

What if I have always been dreaming? That is, what if all of my apparent
perceptual inputs have been generated by my own cognitive system, without my
realizing this? I think this case is analogous to the Evil Genius
Hypothesis: it's just that the role of the "evil genius" is played by a part
of my own cognitive system! If my dream-generating system simulates all of
space-time, we have something like the original Matrix Hypothesis. If it
models just my local environment, or just some macroscopic processes, we
have analogs of the more local versions of the Evil Genius Hypothesis above.
In any of these cases, we should say that the objects that I am currently
perceiving really exist (although objects farther from home may not). It is
just that some of them are constituted by my own cognitive processes.

- Chaos Hypothesis: I do not receive inputs from anywhere in the world.
Instead, I have random uncaused experiences. Through a huge coincidence,
they are exactly the sort of regular, structured experiences with which I am
familiar.

The Chaos Hypothesis is an extraordinarily unlikely hypothesis, much more
unlikely than anything considered above. But it is still one that could in
principle obtain, even if it has miniscule probability. If I am chaotically
envatted, do physical processes in the external world? I think we should say
that they do not. My experiences of external objects are caused by nothing,
and the set of experiences associated with my conception of a given object
will have no common source. Indeed, my experiences are not caused by any
reality external to them at all. So this is a genuine skeptical hypothesis:
if accepted, it would cause us to reject most of our beliefs about the
external world.

So far, the only clear case of a global skeptical hypothesis is the Chaos
Hypothesis. Unlike the previous hypothesis, accepting this hypothesis would
undercut all of our substantive beliefs about the external world. Where does
the difference come from?

Arguably, what is crucial is that on the Chaos Hypothesis, there is no
causal explanation of our experiences at all, and there is no explanation
for the regularities in our experience. In all the previous cases, there is
some explanation for these regularities, though perhaps not the explanation
that we expect. One might suggest that as long as a hypothesis involves some
reasonable explanation for the regularities in our experience, then it will
not be a global skeptical hypothesis.

If so, then if we are granted the assumption that there is some explanation
for the regularities in our experience, then it is safe to say that some of
our beliefs about the external world are correct. This is not much, but it
is something!

------------------------------

9 Philosophical Notes

The material above was written to be accessible to a wide audience, so it
deliberately omits technical philosophical details, connections to the
literature, and so on. Here I will try to remedy this omission. Readers
without a background in philosophy should probably skip or skim this
section.

---

Note 1: Hilary Putnam (1981) has argued that the hypothesis that I am (and
have always been) a brain in a vat can be ruled out a priori. In effect,
this is because my word "brain" refers to objects in my perceived world, and
it cannot refer to objects in an "outer" world in which the vat would have
to exist. For my hypothesis "I am a brain in a vat" to be true, I would have
to be a brain of the sort that exists in the perceived world, but that
cannot be the case. So the hypothesis must be false.

An analogy: I can arguably rule out the hypothesis that I am in the Matrix
(capital M). My term "the Matrix" refers to a specific system that I have
seen in a movie in my perceived world. I could not be in that very system,
as the system exists within the world that I perceive. So my hypothesis "I
am in the Matrix" must be false.

This conclusion about the Matrix seems reasonable, but there is a natural
response. Perhaps this argument rules out the hypothesis that I am in the
Matrix, but I cannot rule out the hypothesis that I am in a matrix, where a
matrix is a generic term for a computer simulation of a world. The term
"Matrix" may be anchored to the specific system in the movie, but the
generic term "matrix" is not.

Likewise, it is arguable that I can rule out the hypothesis that I am a
brain in a vat (if "brain" is anchored to a specific sort of biological
system in my perceived world). But I cannot rule out the hypothesis that I
am envatted, where this simply says that I have a cognitive system that
receives input from and sends outputs to a computer simulation of a world.
The term "envatted" (and the terms used in its definition) are generic
terms, not anchored to specific systems in perceived reality. By using this
slightly different language, we can restate the skeptical hypothesis in a
way that is invulnerable to Putnam's reasoning.

More technically: Putnam's argument may work for "brain" and "Matrix"
because one is a natural kind term and the other is a name. These terms are
subject to "Twin Earth" thought experiments (Putnam 1975), where duplicates
can use corresponding terms with different referents. On Earth, Oscar's term
"water" refers to H2O; but on Twin Earth (which contains the superficially
identical XYZ in its oceans and lakes), Twin Oscar's term "water" refers to
XYZ. Likewise, perhaps my term "brain" refers to biological brains, while an
envatted being's term "brain" refers to virtual brains. If so, when an
envatted being says "I am a brain in a vat", it is not referring to its
biological brain, and its claim is false.

But not all terms are subject to Twin Earth thought experiments. In
particular, semantically neutral terms are not (at least when used without
semantic deference): such terms plausibly include "philosopher", "friend",
and many others. Other such terms include "matrix" and "envatted", as
defined in this article. If we work with hypotheses such as "I am in a
matrix" and "I am envatted", rather than "I am in the Matrix" or "I am a
brain in a vat", then Putnam's argument does not apply. Even if a brain in a
vat could not truly think "I am a brain in a vat", it could truly think "I
am envatted". So I think that Putnam's line of reasoning is ultimately a red
herring.

---

Note 2: Despite this disagreement, the conclusion of this article is closely
related to another suggestion of Putnam's. This is the suggestion that a
brain in a vat may have true beliefs, because it will refer to chemical
processes or processes inside a computer. However, I reach this conclusion
by a quite different route. Putnam argues by an appeal to the causal theory
of reference: thoughts refer to what they are causally connected to, and the
thoughts of an envatted being are causally connected to processes in a
computer. This argument is clearly inconclusive, as the causal theory of
reference is so unconstrained. To say that a causal connection is required
for reference is not to say what sort of causal connection suffices. There
are many cases (like "phlogiston") where terms fail to refer despite rich
causal connections. Intuitively, it is natural to think that the brain in a
vat is a case like this, so an appeal to the causal theory of reference does
not seem to help.

The argument I have given presupposes nothing about the theory of reference.
Instead, it proceeds directly by considering first-order hypotheses about
the world, the connections between these, and what we should say if they are
true. In answering objections, I have made some claims about reference, and
these claims are broadly compatible with a causal theory of reference. But
importantly, these claims are very much consequences of the first-order
argument rather than presuppositions of it. In general, I think that claims
in the theory of reference are beholden to first-order judgments about
cases, rather than vice versa.

---

Note 3: I use "skeptical hypothesis" in this article in a certain technical
sense. A skeptical hypothesis (relative to a belief that P) is a hypothesis
such that (i) we cannot rule it out with certainty; (ii) were we to accept
it, we would reject the belief that P. A skeptical hypothesis with respect
to a class of beliefs is one that is a skeptical hypothesis with respect to
most or all the beliefs in that class. A global skeptical hypothesis is a
skeptical hypothesis with respect to all our empirical beliefs.

The existence of a skeptical hypothesis (with respect to a belief) casts
doubt on the relevant belief, in the following sense. Because we cannot rule
out the hypothesis with certainty, and because the hypothesis implies the
negation of these beliefs, it seems (given a plausible closure principle
about certainty) that our knowledge of these beliefs is not certain. If it
is also the case that we do not know that the skeptical hypothesis does not
obtain (as I think is the case for most of the hypotheses in this article),
then it follows from an analogous closure principle that the beliefs in the
class do not constitute knowledge.

Some use "skeptical hypothesis" in a broader sense, to apply to any
hypothesis such that if it obtains, I do not know that P. (A hypothesis
under which I have accidentally true beliefs is a skeptical hypothesis in
this sense but not in the previous sense.) I have not argued here that the
Matrix Hypothesis is not a skeptical hypothesis in this sense. I have argued
that if the hypothesis obtains, our beliefs are true, but I have not argued
that if it obtains, our beliefs constitute knowledge. Nevertheless, I am
inclined to think that if we have knowledge in an ordinary non-matrix world,
we would also have knowledge in a matrix.

---

Note 4: What is the relevant class of beliefs? Of course there are some
beliefs that even a no-external-world skeptical hypothesis might not
undercut: the belief that I exist, or the belief that 2+2=4, or the belief
that there are no unicorns. Because of this, it is best to restrict
attention to beliefs that (i) are about the external world, (ii) are not
justifiable a priori, and (iii) make a positive claim about the world (they
could not be true in an empty world). For the purposes of this article we
can think of these beliefs as our "empirical beliefs". Claims about
skeptical hypotheses undercutting beliefs should generally be understood as
restricted to beliefs in this class.

---

Note 5: On the Computational Hypothesis: It is coherent to suppose that
there is a computational level underneath physics, but it is not clear
whether it is coherent to suppose that this level is fundamental. If it is,
then we have a world of "pure bits". Such a world would be a world of pure
differences: there are two basic states that differ from one another,
without this difference being a difference in some deeper nature. Whether
one thinks this is coherent or not is connected to whether one thinks that
all differences must be grounded in some basic intrinsic nature, on whether
one thinks that all dispositions must have a categorical bases, and so on.
For the purposes of this paper, however, the issue can be set aside. Under
the Matrix Hypothesis, the computation itself is implemented by processes in
the world of the creator. As such, there will be a more basic level of
intrinsic properties that serves as the basis for the differences between
bits.

---

Note 6: On the Mind-Body Hypothesis: It is interesting to note that the
Matrix Hypothesis shows a concrete way in which Cartesian substance dualism
might have turned out to be true. It is sometimes held that the idea of
physical processes interacting with a nonphysical mind is not just
implausible but incoherent. The Matrix Hypothesis suggests fairly
straightforwardly that this is wrong. Under this hypothesis, our cognitive
system involves processes quite distinct from the processes in the physical
world, but there is a straightforward causal story about how they interact.

Some questions arise. For example, if the envatted cognitive system is
producing a body's motor outputs, what role does the simulated brain play?
Perhaps one could do without it, but this will cause all sorts of awkward
results, not least when doctors in the matrix open the skull. It is more
natural to think that the envatted brain and the simulated brain will always
be in isomorphic states, receiving the same inputs and producing the same
outputs. If the two systems start in isomorphic states and always receive
the same inputs, then (setting aside indeterminism) they will always stay in
isomorphic states. As a bonus, this may explain why death in the Matrix
leads to death in the outer world!

Which of these actually controls the body? This depends on how things are
set up. Things might be set up so the envatted system's outputs are not fed
back to the simulation; in this case a version of epiphenomenalism will be
true. Things might be set up so that motor impulses in the simulated body
depend on the envatted system's outputs with the simulated brain's outputs
being ignored; in this case a version of interactionism will be true.
Interestingly, this last might be a version of interactionism that is
compatible with causal closure of the physical! A third possibility is that
the mechanisms takes both sets of outputs into account (perhaps averaging
the two?). This could yield a sort of redundancy in the causation. Perhaps
the controllers of the matrix might even sometimes switch between the two.
In any of these cases, as long as the two systems stay in isomorphic states,
the behavioral results will be the same.

One might worry that there will be two conscious minds here, in a fashion
reminiscent of Daniel Dennett's story "Where am I"? This depends on whether
computation in the matrix is enough to support a mind. If
anti-computationalists about the mind (such as John Searle) are right, there
will be just one mind. If computationalists about the mind are right, there
may well be two synchronized minds (which then raises the question: if I am
in the matrix, which of the two minds is mine?). The one-mind view is
certainly closer to the ordinary conception of reality, but the two-mind
view is not out of the question.

One bonus of the computationalist view is that it allows us to entertain the
hypothesis that we are in a computer simulation without a separate cognitive
system attached. Instead, the creators just run the simulation, including a
simulation of brains, and minds emerge within it. This is presumably much
easier for the creators, as it removes any worries tied to creation and
upkeep of the attached cognitive systems. Because of this, it seems quite
plausible that there will be many simulations of this sort in the future,
whereas it is unclear that there will be many of the more cumbersome
Matrix-style simulations. (Because of this, Bostrom's argument that we may
well be in a simulation applies more directly to this sort of simulation
than to Matrix-style simulations.) The hypothesis that we are in this sort
of computer simulation corresponds to a slimmed-down version of the
Metaphysical Hypothesis, on which the Mind-Body Hypothesis is unnecessary.
As before, this is a non-skeptical hypothesis: if we are in such a
simulation (and if computationalism about the mind is true), then most of
our beliefs about the external world are still correct.

There are also other possibilities. One intriguing possibility (discussed in
Chalmers 1990) is suggested by contemporary work in artificial life which
involves relatively simple simulated environments, and complex rules by
which simulated creatures interact with these environments. Here the
algorithms responsible for the creatures "mental" processes are quite
distinct from those governing the "physics" of the environment. In this sort
of simulation, creatures will presumably never find underpinnings for their
cognitive processes in their perceived world. If these creatures become
scientists, they will be Cartesian dualists, holding (correctly!) that their
cognitive processes lie outside their physical world. It seems that this is
another coherent way that Cartesian dualism might have turned out to be
true.

---

Note 7: I have argued that the Matrix Hypothesis implies the Metaphysical
Hypothesis and vice versa. Here, "implies" is an epistemic relation: if one
accepts the first, one should accept the second. I do not claim that the
Matrix Hypothesis entails the Metaphysical Hypothesis, in the sense that in
any counterfactual world in which the Matrix Hypothesis holds, the
Metaphysical Hypothesis holds. That claim seems false. For example, there
are counterfactual worlds in which physical space-time is created by nobody
(so the Metaphysical Hypothesis is false), in which I am hooked up to an
artificially-designed computer simulation located within physical space-time
(so the Matrix Hypothesis is true). And if physics is not computational in
the actual world, then physics in this world is not computational either.
One might say that the two hypothesis are a priori equivalent, but not
necessarily equivalent.

(Of course the term "physics" as used by my envatted self in the
counterfactual world will refer to something that is both computational and
created. But "physics" as used by my current envatted self picks out the
outer non-computational physics of that world, not the computational
processes.)

The difference arises from two different ways of considering the Matrix
Hypothesis: as a hypothesis about what might actually be the case, or as a
hypothesis about what might have been the case but is not. The first
hypothesis is reflected in indicative conditionals: if I am actually in a
matrix, then I have hands, atoms are made of bits, and the Metaphysical
Hypothesis is true. The second version is reflected in subjunctive
conditionals: if I had been in a matrix, I would not have had hands, and
atoms would not have been made of bits, and the Metaphysical Hypothesis
would not have been true.

This is analogous to the different ways of thinking about Putnam's Twin
Earth scenario, common in discussions of two-dimensional semantics. If I am
actually in the XYZ-world, then XYZ is water; but if I had been in the
XYZ-world, XYZ would not have been water (water would still have been H2O).
On the first way of doing things, we consider a Twin Earth world as actual.
On the second way of doing things, we consider a Twin Earth world as
counterfactual. We can say that the Twin Earth world verifies "water is
XYZ", but that it satisfies "water is not XYZ", where verification and
satisfaction correspond to considering as actual and as counterfactual.

Likewise, we can say that a matrix world verifies the Metaphysical
Hypothesis, but it does not satisfy the Metaphysical Hypothesis. The reason
is that the Metaphysical Hypothesis makes claims about physics and the
physical world. And what counts as "physics" differs depending on whether
the matrix world is considered as actual or counterfactual. If I am in a
matrix, physics is computational. But if I had been in a matrix, physics
would not have been computational (the matrix would have been computational,
but the computer and my brain would all have been made from
computation-independent physics). In this way, claims about physics and
physical processes in the matrix world are analogous to claims about "water"
in the Twin Earth world.

---

Note 8: The responses to the first few objections in section 7 are clearly
congenial to a causal account of reference. I said that the truth of an
envatted being's thoughts depends not on its immediate environment but on
what it is causally connected to: that is, on the computational processes to
which it is hooked up. As noted earlier, I did not need to assume the causal
theory of reference to get to this conclusion, but instead got there through
a first-order argument. But once the conclusion is reached, there are many
interesting points of contact.

For example, the idea that my term "hair" refers to hair while my envatted
counterpart's term refers to virtual hair has a familiar structure. It is
structurally analogous to a Twin Earth case, in which Oscar (on Earth)
refers to water (H2O) while his counterpart Twin Oscar (on Twin Earth)
refers to twin water (XYZ). In both cases, terms refer to what they are
causally connected to. These natural-kind terms function by picking out a
certain kind in the subject's environment, and the precise nature of that
kind depends on nature of the environment. Something similar applies to
names for specific entities, such as "Tucson".

The behavior of these terms can be modeled using the two-dimensional
semantic framework. As before, when we consider a Twin Earth world as
actual, it verifies "water is XYZ", and when we consider it as
counterfactual, it satisfies "water is not XYZ". Likewise, when we consider
a matrix world as actual, it verifies "hair is made of bits", and when we
consider it as counterfactual, it satisfies "hair is not made of bits".

The difference between considering as actual and counterfactual yields a
perspective shift like the one in the response to objection 5. If the matrix
world is considered as merely counterfactual, we should say that the beings
in the matrix don't have hair (they only have virtual hair). But if the
matrix world is considered as actual (that is, if we hypothetically accept
that we are in a matrix), we should say that the beings in the matrix have
hair, and that hair is itself a sort of virtual hair.

The twin-earth analogy may suggest that the meanings of our terms such as
"hair" and the contents of our corresponding thoughts depends on our
environment. But the two-dimensional approach also suggests that there is an
internal aspect of content that is shared between twins, and that does not
depend on the environment. The primary intension of a sentence is true at a
world if the world verifies the sentence, while its secondary intension is
true at a world if the world satisfies the sentence. Then Oscar and Twin
Oscar's sentence's "water is wet" have different secondary intensions
(roughly, true when H2O is wet or when XYZ is wet respectively), but they
have the same primary intension (roughly, true at worlds where the
watery-looking stuff is wet). Likewise, "I have hair" as used me and my
envatted counterpart has different secondary intensions (roughly, true at
worlds where we have biological hair or computational hair respectively),
but they have the same primary intension (roughly, true at worlds where we
have hair-looking stuff). The primary intensions of our thought and our
language represents a significant shared dimension of content.

---

Note 9: Why the different response to objection 7, on "action" and "friend"?
We noted earlier (note 1) that not all terms function like "water" and
"hair". There are numerous semantically neutral terms that are not subject
to Twin Earth thought-experiments: any two twins using these terms on
different environments will use them with the same meaning (at least if they
are using the terms without semantic deference). These terms arguably
include "and", "friend", "philosopher", "action", "experience", and
"envatted". So while an envatted beings' term "hand" or "hair" or "Tucson"
may mean something different from our corresponding term, an envatted
beings' term "friend" or "philosopher" or "action" will arguably mean the
same as ours.

It follows that if we are concerned with an envatted being's belief "I have
friends", or "I perform actions", we cannot use the Twin-Earth response.
These beliefs will be true if and only if the envatted being has friends and
performs actions. Fortunately, it seems quite reasonable to say that the
envatted being does have friends (in its environment, not in ours), and that
it does perform actions (in its environment, not in ours). The same goes for
other semantically neutral terms: it is for precisely this class of
expressions that this response is reasonable.

---

Note 10: What is the ontology of virtual objects? This is a hard question,
but it is no harder than the question of the ontology of ordinary
macroscopic objects in a quantum-mechanical world. The response to objection
6 suggests that in both cases, we should reject claims of token identity
between microscopic and macroscopic levels. Tables are not identical to any
object characterized purely in terms of quantum-mechanics; likewise, virtual
tables are not identical to any objects characterized purely in terms of
bits. But nevertheless, facts about tables supervene on quantum-mechanical
facts, and facts about virtual tables supervene on computational facts. So
it seems reasonable to say that tables are constituted by quantum processes,
and that virtual tables are constituted by computational processes. Further
specificity in either case depends on delicate questions of metaphysics.

Reflecting on the third-person case, in which we are looking at a brain in a
vat in our world, one might object that virtual objects don't really exist:
there aren't real objects corresponding to tables anywhere inside a
computer. If one says this, though, one may be forced by parity into the
view that tables do not truly exist in our quantum-mechanical world. If one
adopts a restricted ontology of objects in one case, one should adopt it in
the other; if one adopts a liberal ontology in one case, one should adopt it
in the other. The only reasonable way to treat the cases differently is to
adopt a sort of contextualism about what counts as an "object" (or about
what falls within the domain of a quantifier such as "everything"),
depending on the context of the speaker. But this will just reflect a
parochial fact about our language, rather than any deep fact about the
world. In the deep respects, virtual objects are no less real than ordinary
objects.

---

Note 11: The response to objection 8 is reminiscent of the familiar point,
associated with Russell and Kant, that we do not know the intrinsic nature
of entities in the external world. When it comes to physical entities,
perception and science may tell us how these entities affect us, and how
they relate to each other, but these methods tell us little about what the
fundamental physical entities are like in themselves. That is, these methods
reveal the causal structure of the external world, but they leave its
intrinsic nature open.

The Metaphysical Hypothesis is in part as a hypothesis about what underlies
this microphysical causal structure: microphysical entities are made of
bits. The same goes for the Matrix Hypothesis. One might say that if we are
in a matrix, the Kantian ding-an-sich (thing in itself) is part of a
computer-an-sich! This hypothesis supplements our ordinary conception of the
external world, but it does not really contradict it, as this ordinary
conception is silent on the world's intrinsic nature.

---

Note 12: One general moral is that the "manifest image" is robust: our
ordinary conception of the macroscopic world is not easily falsified by
discoveries in science and metaphysics. As long as the physical world
contains processes with the right sort of causal and counterfactual
structure, then it will be compatible with the manifest image. Even a
computer simulation has the relevant causal and counterfactual structure, as
does a process in the mind of God: this is why they can support a robust
external reality, despite their surprising nature.

This sort of flexibility in our conception of the world is closely tied to
the semantic non-neutrality of many of our concepts. Those concepts, such as
"water", "hair", and "electron", leave some flexibility in what their
referent might turn out to be. We conceive of their referents roughly as
whatever actual entity plays a certain causal role, or has a certain
appearance, while leaving open their intrinsic nature. One can likewise
argue that the strongest constraints imposed by our conception of the world
are plausibly those associated with semantically neutral concepts, which do
not yield this sort of flexibility. These concepts plausibly include many of
our causal (and nomic) concepts, as well as many of our mental concepts. In
these cases, we have a sort of "direct" grasp of how the world must be in
order to satisfy the concepts. If so, then our causal and mental beliefs
impose strong constraints on the way the actual world must be.

One can argue that our fundamental semantically neutral concepts are mental
concepts ("experience", "belief"), causal concepts ("cause", "law"), logical
and mathematical concepts ("and", "two"), and categorical concepts
("object", "property"). There are also many semantically neutral concepts
that involve more than one of these elements: "friend", "action", and
"computer" are examples. If this is right, then the fundamental constraints
that our beliefs impose on the external world is that it contain relevant
mental states (in ourselves and in others), and that it contain objects and
properties that stand in relevant causal relations to each other and to the
mental states. This sort of conception is weak enough that it can be
satisfied by a matrix (at least if it is a multi-vat matrix, or if
computationalism about the mind is true).

In my opinion, this issue about the fundamental constraints that our beliefs
impose on the world is the deepest philosophical issue that arises from
thinking about the matrix. If what I have said in this article is right, it
is precisely because these constraints are relatively weak that many
hypotheses that one might have thought of as "skeptical" turn out to be
compatible with our beliefs. And it is this that enables us to mount some
sort of response to the skeptical challenge. A little paradoxically, one
might say that it is because we demand so little that we know so much.

---

Note 13: Why does a computer simulation of a world satisfy these
constraints? The reason is tied to the nature of computation and
implementation. Any formal computation can be regarded as giving a
specification of (abstract) causal structure, specifying the precise manner
of interaction between some set of formal states. To implement such a formal
computation, it is required that the implementation have concrete states
that map directly onto these formal states, where the pattern of (causal and
counterfactual) interaction between these states precisely mirrors the
pattern of interaction between the formal states (see Chalmers 1994). So any
two implementations of the computation will share a certain specific causal
structure. A computational description of the physical world will be
required to mirror its causal structure down to the level of fundmental
objects and properties. So any implementation of this computation will
embody this causal structure (in transitions between implementing states,
whether these be voltages, circuits, or something quite different). So
insofar as our conception of the external world imposes constraints on
causal structure that a real physical world can satisfy, these constraints
will also be satisfied by a computer simulation.

(This relates to a point made by Hubert Dreyfus in his article in this
collection. Like me, Dreyfus takes the view that most of the beliefs of
inhabitants of a matrix will be true, not false. But Dreyfus suggests that
many of their causal beliefs will be false: e.g. their general belief that
"a physical universe with causal powers that makes things happen in our
world", and perhaps their specific beliefs that germs cause disease, that
the sun causes things to get warm, and so on. On my view, this suggestion is
incorrect. On my view, the world of someone living in a matrix has real
causation going on everywhere within it, grounded in the real causation
going on in the computer. Virtual germs in the computer really do cause
virtual disease in the computer. So when a matrix inhabitants say "germs
cause disease", what they say is true.)

Of course the mental constraints also need to be satisfied. In particular,
it is important that the causal structure stand in the right sort of
relation to our experiences. But this constraint will also be satisfied when
we are hooked up to a matrix. Constraints regarding other minds will be
satisfied as long as we are in a multi-vat matrix, or if computationalism
about the mind is true. In this way, a matrix has everything that is
required to satisfy the crucial causal and mental constraints on our
conception of the world.

---

Note 14: A possible line of objection to the argument in this paper is to
argue somehow that there are further constraints that our beliefs impose on
the world that the Matrix Hypothesis does not satisfy. One could argue that
a mere match in mental and causal structure is not enough. For example, one
might argue that the world needs to have the right spatial properties, where
we have some sort of direct grip on what spatial properties are (perhaps
because spatial concepts are semantically neutral). And one could suggest
that the problem with the matrix is that its spatial properties are all
wrong. We believe that external entities are arranged in a certain spatial
pattern, but no such spatial pattern exists inside the computer.

In response, one can argue that these further constraints do not exist. It
can be argued that spatial concepts are not semantically neutral, but
instead are subject to Twin Earth thought-experiments. My student Brad
Thompson has developed thought-experiments of this sort (Thompson 2003),
involving a Doubled Earth where "one meter" refers to (what we call) two
meters, an El Greco World where "square" refers to (what we call)
rectangles, and so on. On this view, our spatial concepts pick out whatever
manifold of properties and relations in the external world is causally
responsible for our corresponding manifold of spatial experiences: in this
respect, spatial concepts are analogous to color concepts. Here we do not
have any "direct" grip on the basic nature of spatial properties. Instead,
once again, the basic constraints are mental and causal.

This line of objection is tacitly engaged in section 9 of the paper, where I
suggest that if there is a computational level underneath physics, then any
implementation of the relevant formal computation could serve in principle
as a realization of that level, without compromising physical reality.
Perhaps an opponent might deny that there could be a computational level
underneath physics, or at least might hold that there are constraints on
what sort of implementation can serve. For example, they might hold that the
implementing level itself must have an appropriate spatial arrangement.

I think that this line of response runs counter to the spirit of
contemporary physics, however. Physicists have seriously entertained the
idea that space as we understand it is not fundamental, but that there is an
underlying level, not described in terms of ordinary spatial notions, from
which space emerges. The cellular automaton hypothesis is just one such
proposal. Here, what is crucial is simply a pattern of causal interaction.
If physicists discover that this pattern is realized in turn by an entirely
different sort of level with very different properties, they will not
conclude that ordinary physical space does not exist. Rather, they will
conclude that space is itself constituted by something nonspatial. This sort
of discovery might be surprising and revisionary, but again no more so than
quantum mechanics. And as with quantum mechanics, we would almost certainly
not regard it as a skeptical hypothesis about the macroscopic external
world. If this is right, then our conception of the macroscopic world does
not impose essentially spatial constraints on the fundamental level of
reality.

Similar issues arise with respect to time. In one respect time poses fewer
problems than space, as the computer simulation in a matrix unfolds in time,
in the same temporal order as time in the simulated world. So one cannot
object that the relevant temporal arrangements are not present in the
matrix, in the way that one could object that the relevant spatial
arrangements are not present. So even if temporal concepts were semantically
neutral, the Matrix Hypothesis could still vindicate our temporal beliefs.
Still, I think one can make a case that our concept of external time is not
semantically neutral (it is notable that physicists have entertained
hypotheses on which temporal notions play no role at the fundamental level).
Rather, it picks out that external manifold of properties and relations that
is responsible for our corresponding manifold of temporal experiences. If
so, then any computer simulation with the right causal structure and the
right relation to our experience will vindicate our temporal beliefs,
regardless of its intrinsic temporal nature.

---

Note 15: The reasoning in this paper does not offer a knockdown refutation
of skepticism, as several skeptical hypotheses are left open. But I think it
significantly strengthens one of the standard responses to skepticism. It is
often held that although various skeptical hypotheses are compatible with
our experiences, the hypothesis that there is a real physical world provides
a simpler or better explanation of the regularities in our experiences than
these skeptical hypotheses. If so, then we may be justified in believing in
the real physical world, by an inference to the best explanation.

At this point is often objected that some skeptical hypotheses seem just as
simple as the standard explanation: for example, the hypothesis that all our
experiences are caused by a computer simulation, or by God. If so, this
response to skepticism fails. But if I am right, then these "equally simple"
hypotheses are not skeptical hypotheses at all. If so, then inference to the
best explanation may work after all: all of these "simple" hypotheses yield
mostly true beliefs about an external world.

The residual issue concerns the various remaining skeptical hypotheses on
the table, such as the Recent Matrix Hypothesis, the Local Matrix
Hypothesis, and so on. It seems reasonable to hold that these are
significantly less simple than the hypotheses above, however. All of them
involve a non-uniform explanation of the regularities in our experiences. In
the Recent Matrix Hypothesis, present regularities and past regularities
have very different explanations. In the Local Matrix Hypothesis, beliefs
about matters close to home and far from home have very different
explanations. These hypotheses as a whole have a sort of dual-mechanism
structure that seems considerably more complex than the uniform-mechanism
structures above. If this is right, one can argue that inference to the best
explanation justifies us in ruling out these hypotheses, and in accepting
the non-skeptical hypotheses above.

Even one thinks that some of these skeptical hypotheses offer reasonably
good explanations of our experience, there is still a promising argument
against global external-world skepticism in the vicinity. If I am right, all
of these skeptical hypotheses are at worst partial skeptical hypotheses: if
they are correct, then a good many of our empirical beliefs will still be
true, and there will still be an external world. To obtain a global
skeptical hypothesis, we have to go all the way to the Chaos Hypothesis. But
this is a hypothesis on which the regularities in our experience have no
explanation at all. Even an extremely weak version of inference to the best
explanation justifies us in ruling out this sort of hypothesis. If so, then
this sort of reasoning may justify our belief in the existence of the
external world.

----------------------------

References

Bostrom, N. 2003. Are you living in a computer simulation? Philosophical
Quarterly 53:243-55. http://www.simulation-argument.com.

Chalmers, D.J. 1990. How Cartesian dualism might have been true.
http://consc.net/notes/dualism.html.

Chalmers, D.J. 1994. A computational foundation for the study of cognition.
http://consc.net/papers/computation.hmtl.

Dennett, D.C. 1978. Brainstorms. In Where am I? MIT Press, 1978.

Putnam, H. 1975. The meaning of "meaning". In Mind, Language, and Reality.
Cambridge University Press.

Putnam, H. 1981. Reason, Truth, and History. Cambridge University Press.

Searle, J.R. 1984. Can computers think? In Minds, Brains, and Science.
Harvard University Press.

Thompson, B. 2003. The Nature of Phenomenal Content. Ph.D. dissertation,
University of Arizona.

Wolfram, S. 2002. A New Kind of Science. Wolfram Media.

----------------------------

The Matrix as Metaphysics
David J. Chalmers
Department of Philosophy
University of Arizona
Tucson, AZ 85721.
[EMAIL PROTECTED]
http://www.u.arizona.edu/~chalmers/papers/matrix.html