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Wilhelm Schelling), energy (Wilhelm Ostwald), sensibilia (Bertrand Russell) or an entangled
indivisible unity of mental and physical aspects, for instance, panpsychism, hylozoism and
panpsychistic identism, (Presocratics, Giordano Bruno, Denis Diderot, Bernhard Rensch). Thus,
we need another condition for a naturalistic theory of mind, e.g., a form of supervenience. Another
reason for that is to exclude the possibility that two different mental states (tokens) are realized by
or identical with one specific physical state (token).
The central intuition of supervenience is that "fixing" the physical fixes everything, or that nothing
could have been otherwise without something physical having been otherwise. Supervenience
holds that for every mental change there must occur a (simultaneous or preceeding) physical
change. Or, to be more precise, a set of mental properties supervenes on a set of physical
properties if and only if any two persons that are indiscernible with respect to their physical
properties are also indiscernible with respect to their mental properties, i.e., if any two persons that
differ with respect to some mental property also differ with respect to at least one physical property.
This is still a crude definition because one must clarify the scope of supervenience (cf.
Beckermann, 1992): Does it hold in every possible world or even between possible worlds? Is it
global or local? If externalism is true we cannot restrict the supervenience of mental events to
neuronal (or bodily) events only, because identical neural events could have different contents in
different environments (cf. II.4.). Thus, local supervenience seems to be too narrow and strong. On
the other hand, global supervenience is much too broad and weak, because it does not seem
plausible that every physical property within the scope of a person's relativistic light cone is relevant
for the mental properties of that person. I do not assume for example that my mental states
supervene on the properties of the thunderstorms in Jupiter's atmosphere or an interaction between
two hydrogen atoms in the Large Magellanic Cloud at this very moment. Of course, this weak or
global form of supervenience does not imply such crazy possibilities, but this example indicates
that something has to be added or specified. Furthermore, supervenience alone is not sufficient for
naturalism and hence a naturalistic theory of mind, because it is also compatible with
epiphenomenalism, parallelism and occasionalism. Thus, we need an identity theory for a
naturalistic theory of mind to exclude such a dualism, or at least a principle of physical exhaustion.
However, as John Haugeland (1984, p. 119) has argued, it might be enough to get rid of
"scientifically unmotivated, magically undetectable, and thoroughly bizarre" hypotheses by shifting
the burdens of proof to the proponents of those hypotheses and accepting the heuristic rule "Don't
get weird beyond necessity".
Nevertheless, fascinating questions and problems remain. For example, are our neural correlates
of consciousness advanced enough to cope with a naturalistic world view (cf. Vaas, 1995b, 1996),
and are they complex enough to understand their own complexity?
Acknowledgements: I am very grateful to Gary Schouborg and Andre Spiegel for valuable comments and
Thomas Zoglauer for discussion. - This paper is based on a lecture given on June 20th 1998 in Bremen
(Germany) at the international conference "Neural Correlates of Consciousness. Empirical and Conceptual
Questions", organized by the Association for the Scientific Study of Consciousness.
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