Why the Difference Between Quantum and Classical Physics
is Irrelevant to the Mind/Body Problem
Department of Philosophy
University of Florida
Gainesville, FL 32611-8545
Copyright (c) Kirk Ludwig 1995
PSYCHE, 2(16), September, 1995
KEYWORDS: Consciousness, eliminativism, emergentism, mind/body problem,
quantum mechanics, reductionism.
COMMENTARY ON: Stapp, H. P. (1995) Why Classical Mechanics Cannot Naturally
Accommodate Consciousness but Quantum Mechanics Can. PSYCHE, 2(5).
ABSTRACT: I argue that the logical difference between classical and quantum
mechanics that Stapp (1995) claims shows quantum mechanics is more amenable
to an account of consciousness than is classical mechanics is irrelevant
to the problem.
1.1 Henry Stapp (1995) argues that "classical mechanics is not constitutionally
suited to accommodate consciousness, whereas quantum mechanics is"
(abstract). This, he asserts, is because of "certain logical deficiencies"
that are not present in quantum mechanics (1.3). The ground advanced for
this claim is that classical mechanics holds that a "physical system
is to be conceived of as fundamentally a conglomerate of simple microscopic
elements each of which interacts only with its immediate neighbors"
(2.12). In particular, a classical description of a system will include
a description of field values at points in the system, but these descriptions
record only what is going on at each of the points, and not features of
the system as a whole. Stapp adds, "One may, of course, postulate
some extra notion of 'emergence'. But nature must be able to confer some
kind of beingness beyond what is entailed by the precepts of classical mechanics
in order to elevate the brain correlate of a belief to the status of an
ontological whole" (2.12). As I understand Stapp, the crucial difference
between quantum mechanics and classical mechanics is supposed to be that
in the latter the basic vector description of the system can include information
about the whole of the system and not just about points in the system: "each
of the 2 x (2L+1)^(M x N)^ registers in the quantum mechanical description
of the computer/brain corresponds to a possible correlated state of activity
of the entire classically-conceived computer/brain" (3.10).
1.2 I contend that this difference between classical and quantum mechanics
is irrelevant to the traditional mind/body problem, of which the problem
of understanding the relation between consciousness and our bodies is one
part. First, I present an account of what that problem is, how it can be
motivated independently of the difference between classical and quantum
mechanics, and what the basic logical options are in responding to it. Second,
I show that the difference between classical and quantum mechanics that
Stapp focuses on does not distinguish between the various logical options
in responding to the problem, and thus that it does not in itself provide
any solution to it. Third, I argue that any solution that is open to the
quantum theorist is equally open to the classical theorist.
2. The Mind/Body Problem
2.1 There are many different mind/body problems, so it would be a mistake
to try to describe 'the' mind/body problem. But the problem I will present
has been a central problem in the tradition in the philosophy of mind, and
is one of the central problems about the relation of mental to non-mental
properties and phenomena, and it is the one in particular with which Stapp
is concerned. The problem is generated by a set of propositions all of which
can be given strong motivations, and yet which are jointly inconsistent.
The problem is that from (2)-(4) we can deduce the negation of (1). That
is the mind/body problem. (The consciousness/body problem is the mind/body
problem restricted to mental properties the possession of which entail that
their possessor is conscious or potentially conscious.)
- Some objects have mental properties.
- The fundamental constituents of objects (i.e., the objects to be listed
in the catalog of particle physics) do not have mental properties.
- Mental properties are not conceptually or definitionally reducible
to non-mental properties.
- Every feature of every object is deducible from a complete description
of it in terms of its fundamental constituents and their properties and
2.2 Responses to the mind/body problem can be classified according to which
of the above four jointly inconsistent propositions they reject. For example,
the Cartesian substance dualist rejects (2), by arguing that there are objects
which have mental properties which are not composite. (This is the ancient
view that the mind or soul is a simple substance.) The panpsychist, who
holds that all non-complex objects have mental properties also rejects (2)
(e.g., Leibniz--see Nagel for a recent discussion). The eliminativist (e.g.,
Churchland) rejects (1). The reductionist (e.g., the functionalist) rejects
(3). Finally, the emergentist rejects (4).
2.3 The problem can be motivated independently of concern for the differences
between classical and quantum mechanics. To motivate (1), we need only the
assumption that we know that we have mental states. To motivate (2), we
need only the following two assumptions. The first is that no basically
new sorts of entities have come into existence since the Big Bang--i.e.,
the list of fundamental physical particles has not changed since the Big
Bang, or, at least, no new sorts of particles have appeared which cannot
be characterized in terms of the same family of properties which we draw
on in describing those originally present.<1>
The second is that at some time in the past no objects had mental properties.
To motivate (3), we can appeal to the apparent conceivability of non-material
thinking beings. Finally, to motivate (4), we can appeal to the success
of science in explaining the behavior of complex systems in terms of laws
governing their constituents. These are all powerful motivations. That is
why the mind/body problem is so hard. But none of these motivations depend
on the features of physical theory that distinguish classical from quantum
mechanics. Thus, we should be skeptical of any claim that a solution to
our problem is to be found by focusing on such differences.
3. Quantum Mechanics and the Mind/Body Problem
3.1 Any solution to the mind/body problem must reject one of the assumptions
that generate it. Our first question is whether the difference Stapp points
to between classical and quantum mechanics shows that if one accepts one
or the other of these theories one must reject one or another of the assumptions
that generate the problem. The answer is 'no' because the difference Stapp
points to, as he acknowledges, is purely a logical one. The difference hinges
on whether the vectors in the basic description of a physical system contain
information only about points or also contain information about the whole
system. Nothing about that difference bears on whether any of the elements
in the vectors entail that the system has mental properties, or that it
does not. So this difference between classical and quantum mechanics makes
no difference with respect to propositions (1) and (2). As a matter of fact,
of course, neither classical nor quantum mechanics assigns mental properties
to the basic constituents of objects. But this has nothing to do with whether
vectors in the basic physical description of a system encode information
only about points in the system or about the system as a whole as well.
The logical difference likewise does not speak to the conceptual relations
between mental and physical properties, and so does not bear on proposition
(3). Nor does the logical difference bear on whether or not the theories
are complete, and so it does not bear on proposition (4). Therefore the
difference which Stapp identifies in the structure of classical and quantum
mechanics is logically irrelevant to the mind/body problem.
3.2 Our second question is whether one of the solutions to the mind/body
problem is more problematic for classical than for quantum mechanics. If
so, then there would still be an important point to focusing on the details
of each for the purposes of resolving the mind/body problem. But precisely
because of the independence of the logical structure of each from the above
four propositions, there is no solution that the quantum theorist could
adopt which the classical theorist could not adopt as well, and vice versa.
3.3 Let us leave aside dualism and panpsychism, which, although logical
options, have little more to recommend them, either on the classical or
quantum mechanical view. That leaves us with eliminativism (rejecting (1)),
reductionism (rejecting (3)), or emergentism (rejecting (4)).
3.4 Clearly, if eliminativism is an option for either view, it is an option
for both. The issue here is basically epistemic. Is our evidence for the
existence of mental state epistemically prior to our evidence that our physical
theories are complete, even in the event that they do not make room for
any mental properties? Since our evidence for any theory in part depends
on our sensory experience, and, indeed, to be in a position to theorize
at all requires having beliefs and desires, eliminativism is self-defeating.
We could embrace it only at the cost of making incoherent our justification
for believing the theories which are supposed to support it. But in any
case it is clear that the issue is independent of the differences between
quantum and classical mechanics.
3.5 What about reductionism? The main contender for an adequate reductionist
theory of mind nowadays is functionalism (see Dennett, e.g.), or a combination
of functionalism with externalism (the view that what thoughts we have depends
on our relations, particularly our causal relations, to things in our environments).
However, quantum mechanics offers no advantage over classical mechanics
here, since the terms required for defining the relevant functional organizations
and relations are no more than those required to describe, e.g., the functional
organization of a computer, and its causal relations to macroscopic objects,
which can be accomplished independently of how our fundamental physics works
out. That leaves conceptual reduction of mental properties to physical properties
at the level of basic theory, but this is equally implausible on either
classical or quantum mechanics.
3.6 The only hope, then, of finding a position that quantum mechanics favors
over classical mechanics lies in emergentism. I assume that emergentism
is in fact the position which Stapp adopts. But emergentism is as much an
option for the classical view as it is for the quantum view. Indeed, emergentism
first became prominent in the latter part of the 19th Century (though, in
fact, both functionalism and emergentism were anticipated by the Ancients).
The point which Stapp seems to think favors quantum mechanics over classical
mechanics here has to do with whether the vectors of the description of
a physical system themselves contain information about the whole system.
But this is relevant only if one thinks that in describing how mental properties
are related to physical properties on the emergentist view one must (a)
treat the emergent property as a property of an aggregate of particles and
(b) treat the emergent property (as seems reasonable) as nomically dependent
on some property of the whole system it is a property of and (c) require
that the property be one information about which can be recovered from that
contained in a vector in the basic physical description of the system. We
can grant the first two requirements, but there is no justification for
the third. Even if a vector description of a classical system does not contain
in each vector information about the whole system, from the complete description
of the system one can of course recover information about the whole system.
There is no conceptual or logical difficulty in an emergent property nomically
supervening on a property ascribed to a system by a description of it in
terms of vectors which record information only about points in or elements
of the system. (A property P supervenes on a family of properties B (B1,...,Bn)
if and only if for any object x, and any property Bi in B, it is nomically
necessary that if x has Bi, then x has P, and for any object x, it is nomically
necessary that if x has P, then x has one of the properties in the family
B. That one property (merely) nomically supervened on another (or family
of others) would not be a fact that emerged from an examination of the physics
of the fundamental constituents of matter.) There is no interesting sense,
then, in which quantum mechanics is more friendly to emergentism than is
3.7 I conclude that the difference between quantum and classical physics
is irrelevant to the mind/body problem.
<1> I am indebted to Scott Hagan for drawing
my attention to the need for this qualification.
Churchland, P. (1981) Eliminative Materialism and the Propositional Attitudes.
The Journal of Philosophy, 78, 67-90.
Dennett, D. (1991) Consciousness Explained. Boston: Little
Nagel, T. (1979) Panpsychism. In Mortal Questions. Cambridge
University Press: Cambridge.
Stapp, H. P. (1995) Why Classical Mechanics Cannot Naturally Accommodate
Consciousness but Quantum Mechanics Can. PSYCHE, 2(06).