February 10, 2005
The identity theory of mind was developed in the 1950s,
chiefly by the philosophers U. T. Place, J. J. C. Smart, and Herbert Feigl.
It is still the historical and conceptual basis for the most prominent
contemporary theories of mind. Versions
of physicalism, materialism, functionalism, and eliminativism – indeed,
virtually every contemporary non-dualist philosophy of mind – developed
directly out of the identity theory. Today,
I’ll take you through the history of its development, so that we can better
understand its underlying conceptual and philosophical structure.
basic idea of the identity theory of mind is easy to define.
It is that each mental state – each wish, desire, sensation,
impression, thought, belief, or idea – is identical with some physical state
of the brain. Every time you or I
are having one of these mental states, our physical brain is in some state, and
the mental state and the brain state are the same thing.
It follows that if we can explain the physical operation of the brain in
terms of the natural laws and principles that are followed by matter within it,
we will have completely explained the operation of the mind as well.
To understand the mind, we don’t have to do anything more than
understand the interactions of physical matter in the mind.
is the basic idea, as I’ve said, of many theories of mind today.
But it was not always a popular position.
Even though versions of physicalism – the view that everything is
ultimately physical in nature – go back to the Greeks, few philosophers at the
beginning of this century thought that anything like the identity theory could
be true. Place, Smart, and Feigl changed all that, partly by answering
standard objections to the identity theory and partly by changing the way
philosophers thought about the problem of explaining the mind.
To understand this, though, we need to begin with Gilbert Ryle’s
influential theory of mind, articulated in 1949 in The Concept of Mind.
his immediately popular and immensely influential book, Ryle sought to give
philosophical explanations of the concepts that we use to discuss and
describe our mental life, concepts like “wishing,” “willing,”
“intending,” “believing,” and “thinking.”
He sought to explain how we use these concepts in ordinary life to
understand ourselves and explain the behavior of others.
Ryle thought that this gave him a powerful alternative to any dualistic
theory of mind. By explaining the
functioning of our concepts of mental explanation, Ryle thought he could avoid
the Cartesian’s assumption of a distinction between the “inner” realm of
the mind and the “outer” realm of behavior and the physical world.
He called the Cartesian theory the dogma of the “ghost in the
machine,” because it held that the physical body is like a machine that is
controlled by a nonphysical, ghostly mind.
did Ryle’s conceptual explanations seek to replace the picture of the “ghost
in the machine” with a more acceptable one?
We can see how his explanations work by going through a simple example.
Consider my wish to have some ice-cream.
Roughly speaking, there are at least two ways to explain this wish. The Cartesian dualist would say that the wish is a state or
aspect of my non-physical mind. It
might cause things to happen in the physical world – for instance it might
cause my body to walk towards an ice-cream shop – but for Descartes, the wish
itself is a property of a completely non-physical thing, the mind.
By contrast, Ryle’s explanation begins by asking what it is that we
mean when we say that somebody wishes to have some ice-cream.
To say that I wish to have some ice-cream is to say things like:
am likely to buy ice-cream within the next few minutes or hours
somebody asks me whether I want ice-cream, I will say “yes.”
somebody gives me ice-cream, I will thank them and eat it.
Australian philosophers U. T. Place and J. J. C. Smart were largely satisfied
with Ryle’s explanations. They
saw that his style of explanation could account for how there could be
distinctively mental concepts even if the world is completely physical, and
there is nothing but physical matter and energy in the world.
Smart, in particular, found Ryle’s explanations “congenial” to the
doctrine of physicalism and to scientific explanation:
seems to me that science is increasingly giving us a viewpoint whereby organisms
are able to be seen as physico-chemical mechanisms: it seems that even the
behavior of man himself will one day be explicable in mechanistic terms.
There does seem to be, so far as science is concerned, nothing in the
world but increasingly complex arrangements of physical constituents.” (p.
explanations could show how this is so in a variety of cases.
For because almost all mental concepts, Place and Smart took it, really
refer to complicated patterns of physical behavior; there is no need to posit a
non-physical mind over and above these patterns of behavior.
to Place in 1956 and Smart in 1959, however, though Ryle’s explanations work
for most mental concepts, there is a special problem that Ryle cannot
solve with certain mental states, states of consciousness or “sensations.”
(This is another way of talking about what contemporary philosophers like
Chalmers find problematic for physicalism – the feeling of “what it’s
like” to be something, qualitative feels, or “qualia.”)
Consider what it’s like to have a tootheache or to see an
afterimage. It seems that there
is something that it’s like for either of these things to happen.
There is something that it is like for us, something it feels like (as it
were) “inside” to have a toothache or see an afterimage.
Having a sensation – for instance having a pain, or having an
afterimage – is having something go on inside, having something happen with
me. The problem with Ryle’s
explanation of the mind is that it must deny this: it must deny, for instance,
that when I have a toothache I am referring to anything real.
For Ryle, to say that I have a toothache is just to say things like:
will probably go to the dentist
will groan and wince in pain
also for Ryle, there is no such thing as the toothache itself.
seemed implausible to Smart, and he proposed a theory that could do better.
On the identity theory, to have a sensation (for instance a pain, an
after-image, or a tickle) is just to be in a certain brain state.
The pain, or tickle, or whatever, just is a brain state: the
mental state and the brain state are identical.
Thus, when I say that I have a tootheache or an afterimage, I am really
saying that I have something: something is going on within me.
this is because I have not thought it out sufficiently, but it does seem to me
as though, when a person says ‘I have an after-image,’ he is making a
genuine report, and that when he says ‘I have a pain,’ he is doing more than
‘replace pain-behavior,’ and that ‘this more’ is not just to say that he
is in distress. I am not so sure,
however, that to admit this is to admit that there ar enonphysical correlates of
brain processes. Whay should not
sensations just be brain processes of a certain sort?”
to Ryle’s theory, there really are pains and afterimages.
But in saying this, we still do not have to posit anything more than
physics: the physical states of the physical brain are all we need.
Thus, we can have physicalism, and also explain consciousness (or so it
this way, the identity theory reconciled physicalism with the natural-enough
idea that talking about sensations is talking about real things.
But the biggest innovation of the form of the identity theory lay in how
it construed the identities it talked about.
For Smart, the identity between mental states and brain states is not
a conceptual identity but an empirical one.
It is not a matter of definition, or because of the structure of our
concepts, that mental states like sensations are brain states.
Rather, this is an empirical fact: something science can discover through
empirical investigation. Science
has often in the past discovered that two things that seemed at first to be
different are actually the same. For
instance, lightning turns out to be the same as a large-scale electrical
discharge. Heat turns out to be the
same as molecular motion. To
take a more familiar and concrete example, “the morning star” is the same
thing as “the evening star.” They’re
both the planet Venus; but we didn’t know this until astronomers discovered
it. We didn’t know these
identities at the beginning of investigation, and they are not built into our
concepts. The meaning of
“lightning” is not, for instance, the same as the meaning of
“large-scale electrical discharge.” Nevertheless,
lightning does turn out to be the same thing, empirically, as large-scale
electrical discharge. The
suggestion of the identity theory is that, similarly, mental states like
sensations and physical states of the brain are empirically identical: upon
scientific investigation, they will turn out to be the same things.
suggestion allows Smart to answer a number of standard objections to the
identity theory. Many of these
objections were familiar and standard: they were the source of the widely held
perception that the identity theory was untenable. In each case, Smart shows how construing the identity theory
as an empirical thesis about what will turn out to be the case, rather
than a conceptual thesis about what is necessarily the case, can give us
the basis for an adequate defense of the theory.
are some standard objections to the identity theory, and the ways that Smart
1: We can know and talk about pains, sensations, toothaches, and
after-images, even if we know nothing about the brain.
This is a problem for the identity theory if the identity theory is
construed as holding that “sensations” means the same thing as
“brain-states.” But it is no
problem if the identity is not conceptual but empirical.
We can know and talk about lightning before we know anything about
electricity or electrical discharge; we can know and talk about the morning star
years before we know that it’s the same thing as the evening star (indeed, we
could know and talk about the morning star even if we never found out what the
evening star is). If the identity
between sensations and brain states is empirical, as Place and Smart say it is,
our ability to know about the one without knowing about the other is no
objection to the identity theory.
2: An after-image can be yellowish-green; but someone looking into the brain
of somebody having this after-image wouldn’t find anything yellowish-green.
Similarly, an after-image is not located in physical space, but
brain-states are. If Place and
Smart are right about the identity theory as an empirical claim, then somebody
who argues this way is in the same position as somebody who argues the
morning star rises in the morning, not at night.
evening star rises at night, not in the morning.
the morning star is not the evening star.
this would be a bad argument. The
morning star is the evening star; we can’t conclude that they’re two
different things just because they show up in two different ways.
If “morning star” meant “evening star,” the argument would be a
good one; but since the identity between the morning star and the evening star
is empirical rather than conceptual, we cannot argue this way.
Similarly, if sensations and brain-states are identical, then we can’t
defeat the identity theory this easily.
other objections, however, are more difficult for the identity theory to deal
Objection: Even if the morning star is the evening star, and even if this is
empirically rather than conceptually true, still, we can refer to it in these
two different ways because it has two different properties: rising in the
morning, and rising in the evening. If
Venus didn’t have these two different properties, we couldn’t refer to it
these two different ways. But if
this is the way things are with “sensations” and “brain-processes,” then
being a sensation really is a different property than being a brain-process.
But now it seems that to explain what’s going on, we have to postulate
different sets of properties: mental ones and physical ones.
In other words, though we can avoid classical dualism – we only have
one kind of stuff – we’re forced to accept property dualism: the view
that there are mental properties of matter that are irreducibly different from
the physical properties of matter.
says that this is the most difficult objection that the identity theory has to
face. Actually, it’s not clear
that the unaugmented identity theory can succesfully respond to it.
Even if mental states are the same as physical states, it seems that we
have to tell a story about why they seem to be so different from physical
states: and if this story just involves saying that, in addition to their
physical properties, they also have mental properties, then we’re back with
answers the objection with a suggestion that actually anticipates the
functionalist theories of mind that would later replace the identity theory as
the mainstream position. On
Smart’s suggestion, to say that one is having a yellow-orange after-image is
not to say that anything has the property of being yellow-orange or even that
anything presents itself to experience as being yellow-orange. It is to say something like: “Something is happening within
me that is similar to what happens when I see a yellow-orange patch on the
wall.” We identify sensations,
not in terms of their own properties, but in terms of their similarities to
other states that we identify by their typical causes and effects.
If this is so, then there are no non-physical “properties” by which
they manifest themselves. In
talking about sensations, we are just talking about brain states in terms of
their typical causes and effects. This
anticipates the functionalist idea that we identify mental states as causal
roles: positions in a network of cause and effect.
fourth objection, though, not considered by Smart, was historically decisive in
showing that the identity theory had to be modified into the position that is
now known as functionalism. The
fourth objection comes from Putnam, who started to express it in the early
1960s. It is called the “multiple
realization” objection or “multiple realizability” argument.
Objection: The identity theory identifies each mental state with a particular
human neurophysiological brain state. Thus, for instance, “pain” might be identified with
“C-fibers firing.” But
organisms that are quite different from humans can be in pain.
Dogs, cats, and horses can be in pain; and it makes sense to imagine that
even a Martian, with neurophysiology that is completely different from human
neurophysiology, could be in pain. Pain
is multiply realizable: therefore it can’t be identical with any human
neurophysiological brain state.
Following the formulation of this objection by Putnam, he and a variety of other philosophers formulated functionalism as a way of accomodating it without denying the root intuition of the identity theory. For the functionalist, to say that something is in pain is to say that it has a state that has a particular place in the causal network: the state is caused by certain stimuli, and it causes certain behavior. This place is the same for anything that is in pain, but what is in this place might differ. For instance, in humans it might be C-fibers firing that realizes pain, whereas in Martians it might be X-fibers firing. So pain itself is neither “C-fibers firing” nor “X-fibers firing,” but it’s whatever occupies the place of pain: whatever is caused by the right sorts of stimuli and causes the right sorts of responses. This can vary from creature to creature, but it could well still be physical in each case. So functionalism is still consistent with physicalism, the view that all that there is is physical matter and properties. But it handles the multiple realizability objection, and also recaptures much of the intuitive motivation that support Ryle’s program to begin with. If functionalism is right, it is true after all, as Ryle thought, that mental states can be identified by reference to patterns of behavior. But the functionalist, following the identity theory, sees a way of making sense of what sensations are that, rather than eliminating the “inner” or construing it as the province of a Cartesian, immaterial, nonphysical mind or self, instead makes it part of the physical world.