Can a machine be conscious? How?
Stevan Harnad
Centre de Neuroscience de la
Cognition (CNC)
Université du Québec à Montréal
CP 8888 Succursale Centre-Ville
Montréal, Québec, Canada
H3C 3P8
harnad@uqam.ca
http://cogsci.soton.ac.uk/~harnad/
Asking whether a machine can be conscious is
rather like asking whether one has stopped beating one's
wife: The question is so heavy with assumptions that either
answer would be incriminating!
The answer, of course, is: It depends entirely on what you mean by "machine"! If you mean the current generation of man-made devices (toasters, ovens, cars, computers, today's robots), the answer is: almost certainly not.
Empirical
Risk. Why "almost"? Two
reasons, the first being
the usual one: (1) "empirical
risk."
We know since at least Descartes that
even scientific "laws" are merely very probable, not
certain. Only mathematical
laws -- which describe consequences that follow provably
(i.e., on pain
of contradiction) from our own assumptions -- are necessarily
true.
But this certainty and necessity are unnecessary for
physics; almost-certainty will do. "Can a particle travel
faster than the speed of light?" Almost
certainly not (at least on current-best theory -- or at
least the last word
of it that trickled down to this non-physicist). "Could a particle travel faster than light?" That
certainly is not provably impossible, though it might be
impossible given certain assumptions. (But those assumptions
are not necessarily correct.)
The Other-Minds Problem.
Empirical risk besets all scientific hypotheses, but let us
agree that it
is not something we will worry about here. There is no need
for roboticists
to be holier than physicists. The second reason for the
"almost" is peculiar
to robotics, however, and it is called (2) the "other minds
problem (Harnad
1991): There is no way to be certain that any other entity
than myself is
conscious (I am speaking deictically: please substitute
yourself for me,
if you too are conscious). This too we owe to Descartes.
In
the long
history of philosophy the other-minds problem has been
puzzled over for
a variety of reasons, usually variants on questions about
what one can and
cannot know for sure. These epistemic questions are
interesting, but we will
not worry about them here, for the usual reason, which is
the following:
It looks on the face of it as if the right strategy for
handling the other-minds
problem is identical to the strategy for handling empirical
risk, namely,
to note that although we can only be 100% certain about two
things -- about (1)
mathematics and about (2) our own consciousness
-- all else being just a matter of probability, some things,
such as scientific
laws and the consciousness of our fellow-human beings, are
nevertheless
so close to 100% sure that it is a waste of time worrying
about them. (Let
us also note, though, that in the empirical science of
robotics, that extra
layer of risk that comes from the other-minds problem might
just come back
to haunt us.)
So let us agree not to worry about the
other-minds problem for now: People other than myself are
almost certainly conscious too, and toasters and all other
human artifacts to date are almost certainly not.
What Is a Machine?
Have we now answered the question "Can a machine be
conscious?" It sounds
as if, at the very least, we have answered the question "Is
any machine
we have built to date conscious?" That makes the original
question sound
as if it was only asking about what we can and cannot build,
which is like
asking whether we can build a rocket that can reach Alpha
Centauri (a rather
vague and arbitrary question about quantitative limitations
on future technology).
But is "machine" defined as "what human beings can
build"? I think
that defines "artifact" -- but "machine"? Or rather, do we
really want to
ask merely: "Can a man-made artifact be conscious?"
And even that would be rather vague, for
"man-made" is itself rather vague. Common sense dictates
that human procreation does not count as "man-making" in
this context. But what about genetic or other biological
engineering? If the day comes when we can craft organisms,
even humans, molecule by molecule, in the laboratory, does
anyone -- or rather, anyone who has agreed to discount the
other-minds problem when it comes to naturally crafted
fellow-humans -- doubt that such a bottom-up construction of
a clone would be conscious too?
So "man-made" is a wishy-washy term. It does
not pick out what we mean by "machine" here. Surely a
toaster (that very same device) would not become more
eligible for consciousness if it happened to grow on a tree
instead of being fabricated by one of us. By the same token,
a toaster would not become any less of a "machine" (whatever that turns out to
mean) by growing on a tree: Two toasters, identical right
down to the last component, one of which I built and the
other of
which grew on a tree, are surely both "machines" (whatever
that means) if
either one of them is. Another way to put this is that we
need a definition
of "machine" that is strictly structural/functional, and not
simply dependent
on its historic origins, if we want to make our question
about what machines
can and cannot do (or be) into a substantive rather than an
arbitrary one.
Kinds of
Machines. But I am afraid that if we
do follow this much
more sensible route to the definition of "machine," we will
find that a
machine turns out to be simply: any causal physical
system, any "mechanism."
And in that case, biological organisms are machines too, and
the answer
to our question "Can a machine be conscious" is a trivial
"Yes, of course."
We are conscious machines. Hence machines can
obviously be conscious.
The rest is just about what kinds of machines can
and cannot be
conscious, and how -- and that becomes a standard
empirical research
program in "cognitive science": The engineering side of
cognitive science
would be the forward-engineering of man-made conscious
systems and the biological
side of cognitive science would be the reverse-engineering
of natural conscious
systems (like ourselves, and our fellow-organisms): figuring
out how our
brains work.
Except for one problem, and it is the one that
risked coming back to haunt us: What does it mean to
"forward-engineer" (or, for that matter, to
"reverse-engineer") a conscious system? It is to give a
causal explanation of it, to describe fully the inner
workings of the mechanism that gives rise to the
consciousness.
Forward- and Reverse-Engineering
the Heart and Brain. Let us take
a less problematic example: To forward-engineer a cardiac
system (a heart)
is to build a mechanism that can do what the heart
can do.
To reverse-engineer the heart is to do the same thing, but
in such a way
as to explain the structure and the function of the
biological heart itself,
and not merely create a prosthesis that can take over some
of its function.
Either way, the explanation is a structural/functional one.
That is, both
forward and reverse engineering explain everything that a
heart can do,
and how, whereas reverse engineering goes on to explain what
the heart is (made out of), and how it in particular
happens to do what hearts can do.
Now let us try to carry this over to the
brain, which is presumably the organ of consciousness.
To forward-engineer the brain is to build a mechanism that
can do what the
brain can do; to reverse engineer the brain is to do the
same thing, but
in such a way as to explain the structure and function of
the biological
brain itself. Either way, the explanation is a
structural/functional one.
That is, both forward and reverse engineering explain
everything that a brain
can do, and how, whereas reverse engineering goes on
to explain what
the brain is (made out of), and how it in particular
happens to do
what brains can do: how it works.
How does the ghost of the
other-minds problem spoil this seemingly straightforward
extension of the
cardiac into the cogitative? First, consider the
forward-engineering: If
we were forward-engineering cardiac function, trying to
build a prosthesis that took over doing all the things the
heart does, we would do it by continuing to add and refine
functions until we eventually built something that was
functionally indistinguishable from a heart. (One test of
our success might be whether such a prosthesis could be
implanted into humans from cradle to grave with no symptom
that it was missing any vital cardiac function.) This
forward-engineered cardiac system would still be
structurally distinguishable from a natural heart, because
it had omitted other properties of the heart -- noncardiac
ones, but biological properties nonetheless -- and to
capture those too may require reverse-engineering of the
constructive, molecular kind
we mentioned earlier: building it bottom-up out of
biological components.
The thing to note is that this cardiac research
program is completely unproblematic. If a vitalist had
asked "Can a machine be cardiac?" we could have given him
the sermon about
"machines" that we began with (i.e., you should instead be
asking "What
kind of machine can and cannot be cardiac, and how?"). Next we could
have led him on through forward-engineering to the compleat
reverse-engineered heart, our constructed cardiac clone,
using mechanistic principles (i.e., structure, function, and
causality) alone. At no point would the cardiac
vitalist have any basis for saying: "But how do we know that
this machine
is really cardiac?" There is no way left (other than
ordinary empirical
risk) for any difference even to be defined ,
because every structural
and functional difference has been eliminated in the compleat
reverse-engineered
heart.
The same would be true if it had been life
itself and not just cardiac function that had been
at issue: If our question had been "Can a machine be alive?"
the very same line of reasoning would show that there is
absolutely no reason to doubt it (apart from the usual
empirical risk, plus perhaps some intellectual or
technological doubts of the Alpha Centauri sort). Again, the
critical point
is when we ask of the man-made, reverse-engineered clone:
"But how do we
know that this machine is really alive?" If there
are two structurally
and functionally indistinguishable systems, one natural and
the other man-made,
and their full causal mechanism is known and understood,
what does it even
mean to ask "But what if one of them is really alive,
but the other
is not?" What property is at issue that one has and the
other lacks, when
all empirical properties have already been captured by the
engineering (Harnad
1994)?
The Animism at the Heart
of Vitalism. Yet this last worry -- "How can we know
it's alive?" --
should sound familiar. It sounds like the other-minds
problem. Indeed, I
suspect that, if we reflect on it, we will realize that it is
the other-minds
problem, and that what we are really worrying about in the
case of the man-made
system is that there's nobody home in there, there is no
ghost in the machine, And that ghost, as usual, is
consciousness. That's the property that we are
worried might be missing.
So chances are that it was always animism that
was at the heart of vitalism. Let us agree to set vitalism
aside, however, as there is certainly no way we can know
whether something can be alive yet not conscious (or
incapable of returning to consciousness). Plants and
micro-organisms and irreversibly comatose patients will
always be puzzles to us in that respect. So let us not dwell
on these inscrutable cases and states. Logic already
dictates that any vitalist who does accept that plants are not
conscious would be in exactly the same untenable
position if he went on to express scepticism about whether
the compleat
artificial plant is really alive as the sceptic about
the compleat
artificial heart (worried about whether it's really a
heart): If there's
a difference, what's the difference? What vital property is
at issue? If
you can't find one (having renounced on consciousness
itself), then you are
defending an empty distinction.
Turing-Testing. But
the same is most definitely not true in the case of
worries about
consciousness itself. Let us take it by steps. First we
forward-engineer
the brain: We build a robot that can pass the Turing Test
(Turing 1950; Harnad
1992): It can do everything a real human can do, for
a lifetime, indistinguishably
from a real human (except perhaps for appearance: we will
return to that).
Let us note, though, that this first step
amounts to a tall order, probably taller than the order of
getting to Alpha Centauri. But we are talking about "can"
here, that is,
about what is possible or impossible (for a machine), and
how and why, rather
than just what happens to be within our actual human
technological reach.
Doing vs. Feeling: The
Feeling/Function Problem. So supposing we do succeed
in building such
a Turing-scale robot (we are no longer talking about
toasters here). Now,
the question is whether he is really conscious: On the face
of it, the only
respect in which he is really indistinguishable from us is
in everything
he can do. But conscious is something I am,
not something I
do. In particular, it is something I feel;
indeed, it is the
fact that I feel. So when the sceptic about that
robot's consciousness
-- remember that he cannot be a sceptic about machine
consciousness in general:
we have already eliminated that by noting that people are a
kind of machine
too -- wants to say that that robot is the wrong kind
of machine,
that he lacks something essential that we humans have, we
all know exactly
what difference the sceptic is talking about, and it
certainly is not an
empty difference. He is saying that the robot does not feel,
it merely
behaves -- behaves exactly, indeed Turing-indistinguishably
-- as if it feels,
but without feeling a thing.
Empirical Robotics.
It is time to remind ourselves of why it is that we agreed
to set aside
the other-minds problem in the case of our fellow-human
beings: Why is it
that we agreed not to fret over whether other people really
have minds (as
opposed to merely acting just as if they had minds, but in
reality being
feelingless Zombies)? It was for the same kind of reason
that we don't worry
about empirical risk: Yes, it could be that the lawful
regularities that
nature seems to obey are just temporary or misleading; there
is no way to
prove that tomorrow will be like today; there is no way to
guarantee that
things are as they appear. But there is no way to act on the
contrary either
(as long as the empirical regularities keep holding).
Empirical risk is only useful and informative
where there is still actual uncertainty about the
regularities
themselves: where it is not yet clear whether nature is
behaving as if it
is obeying this law or that law; while we
are still trying
to build a causal explanation. Once that is accomplished,
and all appearances
are consistently supporting this law rather than that one,
then fretting
about the possibility that despite all appearances things
might be otherwise
is a rather empty exercise. It is fretting about a
difference that makes
no difference.
Of course, as philosophers are fond of
pointing out, our question about whether or not our Turing
robot feels is (or ought to be) an ontic question
-- about what really
is and is not true, what really does or does not, can or
cannot, exist --
rather than merely an epistemic question about what
we can and cannot
know, what is and is not "useful and informative," what does
or does not
make an empirical difference to us. Epistemic factors
(what's knowable or
useful to know) have absolutely no power over ontic ones
(what there is,
what is true).
It would be wise for mere cognitive scientists to concede
this point. Just as it is impossible to be certain that the
laws in accordance with which nature seems to behave are
indeed the true laws of nature, it is impossible to be
certain that systems that behave as if they feel,
truly feel.
Having conceded this point regarding certainty,
however, only a fool
argues with the Turing-Indistinguishable: Yes, the true laws
could be other
than the apparent laws, but if I can't tell the two apart
empirically, I'd
best not try to make too much of that distinction! By the
same token, a robot
that is indistinguishable for a lifetime from a feeling
person might be a
Zombie, but if I can't tell the two apart empirically, I'd
best not try to
make too much of that distinction (Harnad 2000).
Indistinguishable? But surely there are plenty
of ways to distinguish a robot from a human being. If you
prick us, do we not bleed? So perhaps the sceptic about the
forward-engineered robot should hold out for the
reverse-bio-engineered one, the one made out of the right
stuff, Turing-indistinguishable both inside and out, and at
both the macro and micro levels. It is only about that
machine that we can reply to our reformulated question --
"What kinds of machine can and cannot be conscious?" -- that
only that kind can.
But would we be right, or even empirically or
logically justified in concluding that? To put it in a more
evocative way, to highlight the paradoxical polarity of the
"risks" involved: Would we be morally justified in
concluding that whereas the reverse-bioengineered machines,
because they are empirically indistinguishable from natural
machines like us, clearly cannot be denied the same human
rights as the rest of us, the forward-engineered machines,
because they are merely Turing-indistinguishable from us in
their (lifelong) behavioral capacity can be safely
denied those rights and treated as unfeeling Zombies
(toasters)?
Other-Mind Reading and
Turing-Testing.To answer this question we need to look
a little more
closely at both our empirical methodology in cognitive
science and our moral
criteria in real life. Let us consider the second first.
Since at least
1978 (Premack 1978) there has grown an area of research on
what is sometimes called "theory of mind" and sometimes
"mind-reading," in animals and children . This work is not a
branch of philosophy or parapsychology as it might sound; it
is the study of the capacity of animals and children to
detect or infer what others "have in mind." (As such, it
should really be called research on "other-mind
perception.")
It has been found that children after a
certain age, and certain animals, have considerable skill in
detecting or inferring what others (usually members of their
own species) are feeling and thinking (Whiten 1991; Baron-Cohen
1995). The propensity for developing and
exercising this mind-reading skill was probably selected for
by evolution, hence is inborn, but it also requires learning
and experience to develop. An example of its more innate
side might be the capacity to understand facial expressions,
gestures and vocalizations that signal emotions or
intentions such as anger and aggression; a more
learning-dependent example might be the
capacity to detect that another individual has seen
something, or wants something,
or knows something.
Let us note right away that this sort of
mind-reading is a form of Turing-testing: inferring mental
states from behavior. The "behavior" might be both emitted
and detected completely unconsciously, as in the case of the
release and detection of pheromones, or it might be based on
very particular conscious experiences such as when I notice
that you always purse your lips in a certain way when you
think
I have lied to you. And there is everything in between; my
sense of when
you are agitated vs. contented, along with their likely
behavioral consequences, might be a representative midpoint.
Language (which, let us not forget,
is also a behavior) is probably the most powerful and direct
means of mind-reading
(Harnad 1990; Cangelosi & Harnad 2000).
Hence, apart perhaps from direct chemical
communication between brains, all mind-reading is based on
behavior: Turing-testing. It could hardly have been
otherwise. We know, again since at least Descartes, that the
only mind we can read other than by Turing-testing
is our own! As far as all other
minds are concerned, absent genuine telepathic powers (which
I take to be a fiction, if not incoherent), the only
database available to us for other-mind-reading is
other-bodies' behavior.
We do have to be careful not to make the ontic/epistemic
conflation here: The foregoing does not mean that all there
is to mind is behavior (as the blinkered behaviorists
thought)! But it does mean that the only way to read others'
minds is through their behavior, i.e., through
Turing-testing.
Functional- vs.
Structural/Functional-Indistinguishability.
Now, back to our two robots, the reverse-bioengineered one
to whom we were
ready to grant human rights and the merely
forward-engineered one about
whom we were not sure: Both are Turing-indistinguishable
from us behaviorally,
but only the first is anatomically correct. We're all
machines. Is only
the first one the right kind of machine to have a
mind? On what basis
could we possibly conclude that? We have ascertained that
all mind-reading
is just behavior-based Turing-testing, and all three of us
(the two man-made
robots and me) are indistinguishable in that respect. What
else is there?
The rest of the neuromolecular facts about the brain? Which
facts?
There are countless facts about the brain that could not possibly be relevant to the fact that it has a mind: its weight, for example. We know this, because there is a huge range of variation in human brain mass -- from the massive brain of a huge man to the minute brain of a microcephalic, who nevertheless feels pain when he is pinched. Now imagine trying to narrow down the properties of the brain to those that are necessary and sufficient for its having a mind. This turns out to be just another variant of our original question: "What kinds of machines can and cannot be conscious?" We know brains can be, but how? What are their relevant properties (if their weight, for example, is not)? Now imagine paring down the properties of the brain, perhaps by experimenting with bioengineered variations, in order to test which ones are and are not needed to be conscious. What would the test be?
Turing-Filtering Relevant
Brain Function. We are right back to Turing-testing
again! The only
way to sort out the relevant and irrelevant properties of
the biological brain, insofar as consciousness is concerned,
is by looking at the brain's behavior. That is the only
non-telepathic methodology available to us, because of the
other-minds problem. The temptation is to think that
"correlations" will somehow guide us: Use brain imaging to
find the areas and activities that covary with conscious
states, and those will be the necessary and sufficient
conditions of consciousness. But how did we identify those
correlates? Because they were correlates of behavior. To put
it another way: When we ask a human being (or a
reverse-bioengineered robot) "Do you feel this?" we believe
that
the accompanying pattern of activity is conscious because we
believe him
when he says (or acts as if) he feels something -- not the
other way round:
It is not that we conclude that his behavior is conscious
because of the
pattern of brain activity; we conclude that the brain
activity is conscious
because of the behavior.
So, by the same token, what are we to conclude
when the forward-engineered robot says the same
thing, and acts exactly the same way (across a lifetime)? If
we rely on the
Turing criterion in the one case and not the other, what is
our basis for
that methodological (and moral) distinction? What do we use
in its place,
to conclude that this time the internal correlates of the
very same behavior
are not conscious states?
The answer to our revised question -- "What
kinds of machines can be conscious (and how)?" has now come
into methodological focus. -- The
answer is: The kinds that can pass the Turing Test, and by
whatever means are necessary and
sufficient to pass the Turing Test.
Darwin and Telepathy.
If we have any residual worries about Zombies passing the
Turing Test, there
are two ways to console ourselves. One is to remind
ourselves that not even
the Blind Watchmaker who forward-engineered us had a better
way: survival
and reproduction are just Turing functions too: Darwin is no
more capable
of telepathy than we are. So there is no more (or less)
reason to worry that
Zombies could slip through the Turing filter of evolution
than that they
could slip through the Turing filter of robotic engineering
(Harnad 2002).
Turing and Telekinesis.
Our second consolation is the realization that the problem
of explaining
how (and why) we are not Zombies (Harnad 1995)
(otherwise known as
the "mind/body
problem")
is a "hard" problem (Shear 1997), and
not one we
are ever likely to solve. It would be easy if telekinetic
powers existed:
Then feelings would be physical forces like everything else.
But there is
no evidence at all that feelings are causal forces. That is
why our forward-
and reverse-engineering can only explain how it is that we
can do things,
not how it is that we can feel things. And that is
why the ghost in
the machine is destined to continue to haunt us even after
all cognitive
science's empirical work is done (Harnad 2001).
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