Philosophy 4650

Lecture Notes

February 10, 2005


J. J. C. Smart: “Sensations and Brain Processes”


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.

The 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.

This 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.

In 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. 

How 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:

I am likely to buy ice-cream within the next few minutes or hours

If somebody asks me whether I want ice-cream, I will say “yes.”

If somebody gives me ice-cream, I will thank them and eat it. 

And many more ordinary facts and hypotheticals like these.  Ryle’s explanation accounts for the mentalistic concept of “wishing” in terms of ordinary facts and hypotheticals about people, none of which involve referring to an “inner” or distinctively “mental” realm.  For Ryle, what it is to wish for ice-cream (or to desire a new car, or to think that taxes should be raised) is just to satisfy a set of ordinary predictions and statements.  There’s no need to assume that the wish is anything above and beyond the actual and possible verbal and nonverbal behavior that manifests it.  In this way, Ryle sought to eliminate any basis for the Cartesian division of the world into an “inner” and an “outer” realm.


The 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:

“It 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. 61).

Ryle’s 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.

According 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:

I will probably go to the dentist

I will groan and wince in pain

But also for Ryle, there is no such thing as the toothache itself. 

This 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. 

“Maybe 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?”  (pp. 61-62)

Contrary 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 seemed).

In 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.

This 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.

Here are some standard objections to the identity theory, and the ways that Smart answers them:

Objection 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.

Objection 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 following:

The morning star rises in the morning, not at night.

The evening star rises at night, not in the morning.

Therefore the morning star is not the evening star.

But 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.

Two other objections, however, are more difficult for the identity theory to deal with.

Third 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.

Smart 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 property dualism.

Smart 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.

A 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.

Fourth 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.