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>From: chalmers@bronze.ucs.indiana.edu (David Chalmers)
Subject: Re: A rock implements every FSA
Message-ID: <1992Apr18.005812.10584@bronze.ucs.indiana.edu>
Organization: Indiana University
References: <1992Apr17.211851.18106@sophia.smith.edu> <1992Apr17.222658.1055@bronze.ucs.indiana.edu> <1992Apr18.000226.19369@sophia.smith.edu>
Date: Sat, 18 Apr 92 00:58:12 GMT
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In article <1992Apr18.000226.19369@sophia.smith.edu> orourke@sophia.smith.edu (Joseph O'Rourke) writes:

>But can you only conceive of such a world because you do not
>understand physics fully?  (You and everyone else, that is.)
>Your view is only coherent (perhaps) because you don't follow
>out the details of the physics.  It is conceivable (to me) that 
>the existence of matter necessarily implies gravity, that matter 
>without gravity is self-contradictory, in the same sense that a 
>program that can solve the halting problem is self-contradictory.

Well, I have no problem at all conceiving of worlds where different
laws of physics hold, and in a sense the laws of physics in this
world are just irrelevant to that matter.  A world in which the
laws of physics are determined by the rules of Conway's Life on
a 2-dimensional cellular automata is a perfectly coherent possibility,
for instance, and discoveries about the actual laws of physics don't
seem to be able to affect that.

The relevance of discoveries about this-worldly laws of physics seems
only to affect how we will describe those other worlds.  For instance,
it's conceivable that a certain theoretical basis for physics might
make us want to say that the world which one would initially describe
as having matter but not as having gravity in fact does not have matter
at all, due to some intimate theoretical link between the concepts
of matter and gravity.  However, this wouldn't rule out as
impossible the world which we'd imagined -- it would just mean that
we'd have to describe it using a concept other than "matter".

(Compare: the world that we imagine when we say "water is not H20
but XYZ" is a perfectly coherent possibility; it's just that we're
misdescribing it.  A more accurate description would be "a world
in which there's a substance that looks like water, tastes like
water, etc, but whose chemical composition is XYZ".)

>	Can you coherently conceive of a world in which particles can 
>travel faster than light? In which like charges attract?  In which 
>gravity is repulsive?  In which time can flow backwards?  

My initial reaction is to say yes to all of the first three, although
it's just possible that for Kripkean reasons, we wouldn't want to
call the corresponding concepts in those worlds "light", "charge",
and "gravity" (so one would have to say instead "a world in which
there's a force like gravity, except that it's repulsive rather
than attractive").  The fourth may be impossible for deeper conceptual
reasons.

>If you hold this, then it seems you cannot say that any claim about
>gravity is "blatantly false," as we surely do not understand gravity
>today.  It may well be that gravity is necessary in 1992, but we
>just don't know it.  In 10,000 years this may be as evident as
>is the necessity of H2O today.  What you mean is that, according to
>our present abysmal understanding of the physics of gravity today,
>a situation that can only clarify in the future, it is blatantly 
>false that gravity is contingent. This hedges the claim so severely
>that use of the word "blatant" seems inappropriate.

Again, one has to distinguish between the semantically-based
impossibilities and the other ones.  OK, maybe "gravity is
repulsive" could turn out to be impossible, but I see no way
in which "there exists a force like gravity, except that it's
repulsive" could be impossible.  And future discoveries about
gravity are just irrelevant to establishing whether e.g., a
world obeying the laws of Conway's life is impossible.

-- 
Dave Chalmers                            (dave@cogsci.indiana.edu)      
Center for Research on Concepts and Cognition, Indiana University.
"It is not the least charm of a theory that it is refutable."


