Reduction and Elimination in Philosophy and the Sciences

With respect to explanation, there is a huge
difference between contingent and necessary
psychophysical identity. Philosophers who prefer
necessary identity sometimes maintain that we do not
need an explanation, because in their view the existence
of something necessary does not require to be explained.
As their argument goes, there is no sense in asking, e.g.,
why squares are rectangles, because the opposite cannot
be the case. The same would go for psychophysical
identity: if F were necessarily identical to G, it could not be
otherwise; therefore we would not need to explain why F is
identical to G. I think this argument is almost correct. It
only needs to be added that there is a reasonable answer
to the question why squares are rectangles, even though it
is quite trivial: squares are rectangles because the words
"square" and "rectangle" are used in a certain way; more
precisely, because the word "square" is defined in such a
way that it refers to rectangles with equal sides. In a similar
vein, we could answer the question why a necessary
psychophysical identity exists: because F and G (or, for
that matter, the predicates these symbols stand for) are
used in such a way that both refer to the same entity. In
short, the existence of a necessary identity can be
explained by how the language is used.
An explanation like this, however, is not sufficient in
the case of contingent identity. A contingent identity cannot
be explained by language use alone, although the way the
words are used always contributes to the explanation of
why a sentence is true. A sentence has its truth value
partly because of the meanings of its components. But in
the case of contingent truths this cannot be the whole
explanation. That an identity statement is contingently true
means that it could also be false. Why is it not false? Why
doesn´t the identity not exist? If we could answer these
questions solely by how the language is used, the
statement "F is identical to G" would be true in all possible
worlds in which the words are used as they are used in the
actual world. And this would contradict the assumption that
the identity statement is contingently true. Therefore we
need an explanation that goes beyond language use.
Obviously, we can exclude causal models of
explanation which are used in some branches of dualism
but are not suitable for explaining psychophysical identity.
In particular, it does not make sense to say that a mental
state is caused by the corresponding brain state, if the two
are identical. An effect must be different from its cause.
So, what we look for is an explanation of psychophysical
identity that goes beyond language use and does not rely
on causal interaction. Which alternatives are left? It is
useful to consider examples of identities and their
explanations in other areas, where we find at least three
different explanatory models.
4. Three Models of Explaining Contingent
Identity
The first one consists in analyzing identity in terms of nomological
connections. Suppose two different things, A
and B, are united by some physical process, like two drops
of water that fuse into one when touching each other. Regarding
identity, this process allows for different descriptions.
An interesting way of describing it is to say that A
and B, not having been identical before the fusion, are now
identical. Let us accept this description for a moment, just
to have an identity that we can explain. We can do this by
reference to the fact that the molecules of drop A are now
connected to the molecules of drop B by physical forces
which can be described by laws of nature. What we call an
identity is in fact a set of nomological connections.
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Problems with Psychophysical Identities — Peter Kügler
The second kind of identity, and the second kind of
explanation, is based on definite descriptions that refer to
the same thing. Suppose a single person wrote the Iliad
and the Odyssey. If this is true, the author of the Iliad was
identical to the author of the Odyssey. It is easy to
understand how both descriptions can refer to the same
thing: Homer wrote both books during his lifetime. We
know what a human being is, and we also know how a
single human being can write two books.
The third and last kind of identity explanation
concerns epistemic perspectives. Take any of the familiar
examples of the relativity of perception, say, Locke´s
example of the water that feels warm to one hand and cold
to the other (because the one hand has been cold and the
other has been warm before immersion). The water is the
same for both hands, and so is its temperature,
understood as the kinetic energy of the water molecules.
The sensations of warm and cold are two perceptual
perspectives on the same object. Three things are
involved: a sensation of warm, a sensation of cold, and the
temperature itself, which is neither warm nor cold in a
sensational sense.
To conclude, in order to understand identity, we may
refer to nomological connections, to definite descriptions or
to epistemic perspectives. When applying these three
models to psychophysical identity, we realize that they
correspond to well-known positions in the philosophy of
mind. The first one is represented by parallelism: F and G,
the mental and the physical, form a unity, being connected
to each other by psychophysical laws. In the history of
philosophy the existence of these laws has sometimes
been traced back to God, e.g. by Leibniz, but this is no
necessary part of the theory. We can also stop the
explanatory regress at the laws themselves.
The second model suggests a double-aspect view,
which is often confused with parallelism. What I mean,
however, is the idea that F and G are but two aspects of
reality which has other aspects too. As I understand the
two theories, these other aspects make the difference
between double-aspect theory and parallelism. Compare
this to the example of Homer: if he was a real person, he
was not only the author of the Iliad and the Odyssey, but
also had other properties characteristic of real persons. He
had a heart and a brain, was born and died, and so on. In
analogy, the double-aspect theory conceives of F and G
as two aspects of a larger whole with additional properties.
It is often assumed that these properties are unknown or
even unknowable to us, just like we do not know many of
the properties of Homer. But nevertheless we may
speculate that we would understand how F and G are
linked to each other if we knew the other properties of the
psychophysical whole.
Neutral monism, which is our third position, regards
F and G as two epistemic perspectives on reality which in
itself is neither mental nor physical. As its name indicates,
neutral monism rejects physicalist monism. Of course, the
same is true of parallelism and the double-aspect theory,
but while these are varieties of dualism (or rather pluralism
in the second case), neutral monism is neither physicalist
nor dualist. At least this is the intention behind the theory.
These considerations suggest that the quest for an
explanation of contingent psychophysical identity leads to
non-physicalist models, either to a kind of non-monism
(parallelism, double-aspect view) or to a non-physicalist
(neutral) monism. Of course, we may still raise the
question whether parallelism, double-aspect theory and
neutral monism are really non-physicalist views. After all,
each of them has also been interpreted as a kind of