In the
philosophy of mind, the question of the
relationship between the
physical and
mental is
at issue. The
mind-brain identity theory
holds that the relation in question is in fact the
identity relation and that the brain
(and possibly other parts of the body, like
the
CNS) literally
is the mind.
To a scientific outlook, this can look like a
straightforward and obvious statement of fact.
Given that minds are real (we hope this much
is apparent, even to hard-line materialists)
and that if we look with our scientific
instruments we find only
physical stuff, it
is convenient and simple to identify the mind
with the physical stuff that we find when we
go looking for it scientifically.
But this view does not mesh straightforwardly
with our commonsense notions and our
normal linguistic practices,
hence the identity theory has been the subject
of much controversy. Nonetheless, it has been
extremely influential, and most current
discourse in analytic philosophy on the
ontology of the mental has been shaped by its
defence, development or opposition.
In the ancient world, Epicurus could
be considered a kind of identity theorist, but
in its modern formulation, the originators of
the theory were Herbert Feigl1,
J.J.C. Smart and U.T. Place.
Place took the view that phenomena such as
after-images, though real, can't be
accounted for in a dispositional (ie.
behaviourist) analysis of the mental (which
he favoured in the case
of concepts like belief and intention.)
Finding untenable the traditional dualist
view of mental events as distinct from, though
correlated with, physical events, he
was led to postulate the identity of
the former with the latter. His view was
that this identification was a scientific theory,
in the same way that the identification of lightning
with the motion of electrical particles is
not a necessary truth, but a contingent one that
we establish empirically.
The theory was expanded by D.M. Armstrong in
his influential book A Materialist Theory
of Mind where it was elevated to a fully
fledged ontology. Roughly, Armstrong's view
is that all real properties are causally based;
he called this the causal theory of properties
or CTP for short.
Thus, if we want to say that 'mental properties'
(like is seeing a red after-image, for example)
are real, then unless we wish to invent some
non-physical variety of causality, which Armstrong
certainly did not want to do, we have to
postulate some corresponding physical property
of the brain, in virtue of which our claim to
be seeing a red after-image can be thought true.
But, as was pointed out, we need to be clear
whether we are identifying mental
properties with physical ones or just individual
mental events and states with physical ones.
A theory which supports the view that mental
properties are real and identical with physical
ones is called a type-type identity theory,
as it asserts there are types of mental events
which are identical with types of physical events.
The more hard-line token-token identity
theory, by contrast, is one in which we assert
the identity of particular, single, mental
states and events with particular single physical
ones, but deny that there are necessarily any
real physical properties corresponding to the
terms we use to classify the mental (terms such
as after-image, for example.)
The type-type theories, then, are realist about
our descriptions of the mental (or at least they
allow realism about these) whereas the token-token
theories are not: propositions about
the mental can be seen, at best, as useful
approximations to the true physical description,
or as simply denoting arbitrary disjunctions
of physical objects.
This has obviously travelled some distance from
the identity theory as originally conceived, which
sought to preserve the potential truthfulness of
statements about the mental, and the token-token theory
has been criticised for simply restating
epiphenomenalism - the theory that mental
events are some strange metaphysical exudence
of physical ones - in a hidden guise.
But if, adopting the type-type approach, we say an after-image is identical to a process in the brain, then since the after-image is green we would have to say that the brain-process is green, since identicals share all properties. This objection against the type-type theories is regarded by many as fatal.
Donald Davidson has put forward a theory
known as anomalous monism in which, though
only token-token identity holds between the mental
and the physical, there are nonetheless real
physical and mental properties, but with no
type-type-like correlation between them.
My own view is that it's the materialist, or
physicalist stance that's leading us
into trouble here. I am not inclined to say that
there are two of me here, a physical one and a
mental one, but if we adopt Armstrong's
CTP, together with his view (shared, I think,
by most identity theorists) that 'causal'
means 'investigable by physics', then it seems
quite clear that descriptions that are acceptable
in physics will not exhaustively describe
our experience (see inverse spectrum argument.)
So I am led to postulate a
'dual aspect' version
of neutral monism, instead.
The most significant current objection to
the identity theory, however, is widely thought to be
Hilary Putnam's multiple realizability
argument, which considers that it would be
possible for many different physical states to implement
a single mental state, and since the mental
states are identical to each other and the
physical ones not, the mental states can't be
identical with the physical ones. This consideration
has probably been influential for those
who espouse computationalist
theories of mind on the analogy that many different physical
arrangements can realise identical computations.
1. see nomological dangler.