Reduction and Elimination in Philosophy and the Sciences

Problems with Psychophysical Identities
Peter Kügler, Innsbruck, Austria
1. Theories of Psychophysical Identity
The idea of psychophysical identity is sometimes expressed
by saying that the mind is identical to the body, or
to one of its parts, the brain. This could mean different
things. If it is meant as a rejection of the Cartesian dualism
of mental and physical substances, it would be better to
say that minds, i.e., mental substances do not exist at all.
Here, elimination seems to be more appropriate than identification.
If Descartes is wrong, souls are not identical to
the body, they simply do not exist.
But of course, other kinds of mind-body identification
are available. Type identity theory says that mental types
are identical to physical (neurophysiological) types. In
different versions of this theory the types are conceived as
types of properties, events, processes, or whatever the
preferred ontology is. Token identity theory identifies
mental tokens with physical tokens without assuming
identity of types. And functionalism, as it is usually
presented, identifies mental types with functional types
which are said to be "realized" by the physical properties of
the brain. It is common to note that functionalism is
actually ontologically neutral, because the functional
properties could also be realized by non-physical
properties. But it is no less common to supplement the
functionalist framework with physicalist assumptions.
Functional types are defined by their causal roles, which
include physical causes and physical effects. Physicalists
assume that these causal relations are describable by
physics. Given this additional premise, the property that
realizes the functional type must be a physical property. In
this way, functionalism leads to psychophysical identity;
whether it is type identity or token identity does not matter
in the present context.
Type identity theory, token identity theory and
physicalist functionalism identify the mental with the
physical. Hence they aim at reducing the mental to the
physical in one sense of "reduction". In the following I will
try to argue against these three theories. For ease of
discussion, I will use the open sentence "F is identical to
G" for psychophysical identity statements of all sorts. F is a
psychological and G a physical term suitable for the
respective theory. So the terms may either refer to types or
to tokens.
2. Psychophysical Identity as Necessary
Suppose that F is identical to G. Whether F and G stand
for types or tokens, there are two possibilities as to the
nature of the respective identity statements. They are either
meant to express a necessary or a contingent truth.
Let us consider the first option first and suppose that "F is
identical to G" is necessarily true. It is well known that this
assumption flies in the face of anti-physicalist arguments
based on modal considerations. Descartes famously argued
that he is not a material substance, because
whereas he cannot doubt the existence of his mind, his
body might not exist. The mind could exist without the
body, therefore the two are not identical. Contemporary
philosophers have criticized psychophysical identity theories
by assuming that in other possible worlds mental
properties are correlated with different physical properties
than in the real world, or by evoking a Zombie world that is
physically identical to the real world but contains no consciousness
at all. Arguments like these rely on the possible
dissociation of the mental and the physical. It is important
to keep in mind that they only work against psychophysical
identity claims assumed to be necessarily true.
When type identity theory was put forward in the
1950s, it was meant as a theory of contingent identity; as
an empirical discovery that need not necessarily be true.
But today most philosophers seem to be convinced that
type identity implies necessary coextension, which means
that a mental and a physical type can only be identical if
they are correlated in all possible worlds. If this is correct,
type identity is seriously threatened by modal
counterarguments. But these arguments threaten token
identity too. Suppose a mental token F is necessarily
identical to a physical token G. There seem to be possible
worlds in which F is correlated with different physical
tokens, or with none at all. Conversely, there seem to be
possible worlds in which G is correlated with different or no
mental tokens. For example, we can imagine a possible
world that is exactly like ours except that my current visual
impression is correlated with a different brain state, or my
current brain state with a different visual impression. So
token identity does not look like a necessary relation
either.
A possible defence against these and similar
objections to psychophysical identity is to challenge the
underlying modal intuitions. The objections presuppose
that we are able to imagine, to think about, or to
consistently describe possible worlds that differ from ours
in containing other psychophysical correlations, or even
Zombies without minds. But our imagination is limited, and
so are our thoughts and descriptions, or any other faculty
that is supposed to provide epistemic access to possible
worlds. Thus there are reasons to be sceptical about the
reliability of possible-world arguments. But although it is
good to be cautious, one must add that there is no better
access to possible worlds than that provided by
imagination, thought, and description. When investigating
such alternatives to reality we depend on these methods of
metaphysical enquiry. Without them, it would be pointless
to claim that psychophysical identity exists necessarily. If
you dismiss imagination, thought and description as
insufficient methods for exploring possible worlds, you
must also dismiss the necessity of psychophysical identity.
3. Explaining Psychophysical Identity
Since necessity seems to be a dead end, I will now turn to
the assumption that psychophysical identity is a contingent
truth. In this perspective, questions of explanation become
particularly important. Suppose a psychophysical identity
claim is true. Do we need to explain why it is true? And, if
so, how can we explain it? Is there an answer to the question
"Why is F identical to G?" And do we need such an
answer to understand that identity? To preclude a possible
misunderstanding: these closely related questions aim at
explanation, not at epistemic justification. A justification for
the claim that F is identical to G might consist in empirical
evidence that F and G are regularly correlated. But even if
such an observed correlation were a good reason for believing
that F and G are identical, it would not explain why
this identity exists.
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