every last microscopic particle in the universe has beenWxed, there is
simply noroomfor any further variation (except by conservative addition
- Chalmers allows that a world might diVer from ours in having some-
thingextra, such as angels constituted out of non-physical ectoplasm,
provided that they make no diVerence to the distribution of micro-
physical particles).
In contrast, Chalmers claims, phenomenal consciousness does not
supervene logically on the physical world. For it is easy to imagine a world
which is micro-physically identical to ours, but in which there is nothing
which it feels like to be one of the organisms (including the human beings)
in that world. This is thezombieworld. And it is the conceivability of
zombie worlds (and/orinverted qualiaworlds – see below) micro-physical-
ly identical to our own which makes the problem of phenomenal con-
sciousness sohard, Chalmers thinks – indeed, which makes it insoluble
from within a physicalist and/or functionalist framework.
Chalmers also maintains that only those natural properties which
supervene logically on physical ones can admit of any sort of reductive
explanation. (By reductive explanation he means explanation by instan-
tiation in, or composition by, lower-level mechanisms and processes; so
reductiveexplanationis to be distinguished sharply from ontological or
theoretical reduction, of the sort discussed above in chapter 7, section 5.)
According to Chalmers, our concept of any given higher-level process,
state, or event, speciWes the conditions which any reductive explanation of
that phenomenon must meet. For example, our conceptlifecontains such
notions asreproductionandenergy generation by metabolic processes,
which are amongst the functions which any living thing must be able to
perform. And then a reductive explanation of life will demonstrate how
appropriate chemical changes and processes can constitute the perfor-
mance of just those functions. The phenomenon of life is explained when
we see how those lower-level chemical events, suitably arranged and
sequenced, will instantiate just those functions which form part of our
conceptliving thing. In fact it is science’s track-record of success in provid-
ing such reductive explanations which warrants our belief that physics is
closed in our world, and which provides the grounds for the claim that all
natural phenomena supervene on micro-physical facts.
Since concepts of chemical, geological, geographical, meteorological,
biological, psycho-functional and economic states and processes are
broadly functional ones, it is possible for events of those kinds to admit of
reductive explanations. But our concepts of phenomenally conscious
states are diVerent, as evidenced by the conceivability of zombie (and
inverted qualia) worlds. If we can conceive of states which are functionally
identical to our conscious experiences while being phenomenally distinct,
240 Consciousness: theWnal frontier?