Physicalism

Everybody acknowledges that there are some very tight links between mental states and physical states of the brain, but how could we go the extra step and show that mental states are identical to physical states?
 

I want to sketch what is probably the most influential argument for a version of Physicalism.
 

1    The causal theory of mental concepts

Causal concepts
Consider the concept of poison:  something is poison if it brings about illness or death when ingested. Bringing about illness or death specifies a causal role which a substance may or may not be capable of playing.  Any substance which does play that causal role is poison.   Now consider a more complex causal role: that associated with being manufactured poison.  To be a manufactured poison a substance must not only bring about illness, but also it itself must be brought about in a certain way.  To be a manufactured poison a substance must occupy a certain causal niche, a causal role.  The concept of manufactured poison is exhausted by a specification of that causal role.  However, which substances actually play that causal role cannot be ascertained from an examination of the concept of poison.  In order to find out what substances play the causal role, and which do not, one must undertake empirical research:  one must find out which substances cause death or illness when ingested, and so on.

The causal theory of mental concepts.
The causal theory of mental concepts claims that mental concepts (like the concept of poison) are all causal.   What is pain?  Pain is a mechanism for warning an organism that its body is damaged and to take appropriate action, so that it is an inner state of a person which is (typically) brought about by bodily damage and which (typically) brings about pain-behaviour.  When we see somebody whose body is damaged, and who is writhing and groaning, we say 'he is in pain'.  But we do not simply mean that he is exhibiting pain behavior.  Rather, we are conjecturing the presence of an inner mental state of the person (pain) which is brought about by the bodily damage, and which brings about the pain-behaviour.   Using arrows to indicate causal links, and X to indicate the position of pain in the causal sequence, we can depict the causal role like this:

 bodily damage Æ X Æ pain behavior.

Whatever actually fills the gap marked by X is  pain.

Other mental concepts, so the theory goes, can be analyzed in a similar way.

The interesting feature of this account of mental concepts is that it leaves open the nature of the inner states which play the causal roles.  For all this analysis says, the mental state of pain may be a non-physical state of a Cartesian mind, or it may be a physical state of the central nervous system.  Which one it is will depend on the way the world is constructed, and will not itself be open to purely philosophical investigation.  In order to find out what pain is, for example, we have to do some hard empirical research.

Mental/physical identity as a scientific hypothesis:
Suppose that scientists discover that stimulation of a certain part of the brain, say the C-fibres, plays the causal role associated with pain. Bodily damage typically brings about such stimulation, and such stimulation, via the motor center in the brain, brings about typical pain behavior.  That is, we can fill in the X position in the diagram above:

 bodily damage Æ C-fibre stimulation Æ pain behavior

Since whatever fills the gap previously marked by X is pain, we would have an argument that pain just is the stimulation of C-fibres.

Note that this argument for materialism requires two very different kinds of premises:
 

          (i) A philosophical premise: an analysis of the the concept of pain;

 and   (ii)  A scientific premise: a piece of empirical research into the brain.

 A “dualist” about genes might say:  'No. there are two different things here, the genes, and the DNA molecules, and the scientists have simply discovered that there is a correlation.  Genes are merely correlated with DNA molecules: they are not identical to DNA molecules.'  Of course there are no such “gene-DNA dualists”.  The position seems an absurd one, and the absurdity of the position is meant to make the mind-body dualist similarly uncomfortable.  For the mental/physical dualist might say: 'No, pain is not identical to C-fibre stimulation.  There are still two things, the pain and the stimulation of C-fibres:  it is just that there is a one-to-one correlation between these two things: whenever one occurs the other occurs.'  While this may initially sound plausible in the case of pain and the associated brain state, it does not sound at all plausible in the case of genes and DNA.  We want to say that DNA molecules just are genes.  There are not two different things in the world (genes and DNA molecules) which happen to be 'correlated', but just one thing: DNA molecules playing a certain causal role.

Some more analogies:  Lightning was discovered to be just an electrical discharge in the atmosphere; water a collection of H2O molecules.  Consider lightning.  What is lightning?  It is that stuff which is brought about in certain sorts of stormy conditions and which causes us to see great streaks of light in the sky, starts fires, and so on.  It was later discovered that the stuff that does this is an electrical discharge in the atmosphere.  So, there are not two things, lightning, and an electrical discharge with which the lightning happens to be contingently correlated, but just one thing.  The lightning is the electrical discharge.

What about water?  This is more tricky, because water is known through very many different causal roles.  Water, we might say, is that stuff that you typically find in rivers, and oceans, has a certain appearance, and sustains various life processes.  The stuff that does all that is, as it happens, collections of H2O molecules.  So there are not two sorts of things floating about, collections H2O molecules and water (which happens to be correlated with water).  Water just is a collection of H2O molecules.
 

2    Advantages of the mental-physical identity theory

-    There is no problem of interaction:  all the states that do any causing are physical states.  Mental states can interact unproblematic ally with physical states, both as effects and causes.  There is no problem about, for example, bodily damage causing pain. Supposing that stimulation of C-fibres plays the appropriate causal role, then that state is pain, and there of course there is no particular problem why bodily damage should bring about pain (=the stimulation of C-fibres).  Similarly there is no particular problem about the causal efficacy in pain in bringing about changes in the body (possibly via other mental states, which will themselves be identical to certain states of the brain).

-    There is no problem of locatability.  Mental states will probably turn out to be spatially rather diffuse states of the central nervous system.  So diffuse, perhaps, that we would be unwilling to pinpoint their spatial location.  But this is a problem of locating any diffuse purely physical state as well.  There is no particular problem of the locatability of the mental.

-  Finally, the mind-brain identity theorist can counter some of the arguments from possibility without conceding dualism.  It is logically possible that my mind survive the death of my body, in the sense that it is logically possible that my mind be a distinct thing from my body.  However, as it happens my mind is my central nervous system, that is, it is apart of my body, and (if this is right) the death of the body will bring about the death of the mind.  Similarly it  is logically possible that my mind occupy a completely different body, though not logically possible that my body occupy a completely different body (because it is logically possible that my mind be a distinct thing from my body).