From: chalmers@bronze.ucs.indiana.edu (David Chalmers)
Subject: Re: Dualism
Organization: Indiana University
Date: Thu, 10 Sep 1992 20:15:58 GMT

In article <18ng7rINNbp3@agate.berkeley.edu> epfaith@sunkist.berkeley.edu (Edward Paul Faith) writes:

>But I think that it also is worthwhile to explain why anyone would
>say such a false thing.  It turns out that there are two quite separate
>senses of the word "possible", and according to one sense, it is
>indeed true that "What I can imagine is possible."
>
>The first use is objective, and descriptive.  It is possible to use the
>human hand to manipulate things.  This is the notion that is
>important for the dualist argument, because a soul is shown to be
>separate from matter if it is inherently possible for it to be
>disembodied.

Well, it's important to separate two different kinds of "objective
possibility".  The first, which seems to be the one you're referring
to here, is mere nomic possibility, i.e. possibility subject to
the laws of nature.  Another, which is typically more relevant to
these arguments, is logical possibility, which one can try to
explicate via conceptual coherence, or via the existence of some
possible world that makes a proposition true, although there is
some circularity in all these explications.  It's complicated
further by the fact that certain aspects of logical possibility
are a posteriori, a la Kripke, but leave that aside for now.
What's important here is that the laws of nature aren't much
of a constraint on logical possibility.  It's logically possible
that pigs can fly, but nomically impossible (unless they're in an
airplane).

>The other use is subjective.  It is possible for all you know that I
>have brown eyes.  And yet to one of my friends it may be
>impossible.  Possibility is descriptive not of my eyes, but of your
>relationship to my eyes.

This is mere epistemic possibility, and presumably not much
metaphysics can turn on this (though lots of epistemology might).

>Now, let us look at " What I can imagine is possible." in this
>light.  It turns out that it is true when we use "possible" subjectively.
>But it is false when we use "possible" objectively.  So the whole
>argument is in fact trivially true if we use "possible" subjectively.
>But then it is not useful to dualists.

Well, I think "imagining" is a very vague concept, but seems to me
that there is a sense in which what one can imagine is logically
possible.  This is subject to a number of constraints: (1) one
must be fully imagining the situation, and all the truth-makers
for the relevant proposition -- one can't just imagine being
in a position where one would have justification for believing
the proposition.  In the latter sense, one could e.g. imagine that
Godel's Theorem was false (as one can imagine being in a situation
where one has just proved the decidability of arithmetic), but in
the strict sense, I don't think one can imagine it at all, as there's
no possible world in which it's true.  (2) There are a posteriori
constraints on logical possibility, when empirical discoveries
lead us to constrain the application-conditions of a concept,
such as water (it's an a posteriori conceptual necessity that
water is H20), or Hesperus and Phosphorus.  This means that
one can think one is imagining something when really one isn't
(you think you're imagining that water is XYZ, but actually
you're imagining something that looks like water but isn't).  These
things complicate arguments from logical possibility, but don't
render them vacuous.  In general, however, it's a lot easier
if one skips the "imagination" step entirely.

That being said, it's certainly false that what one can imagine
is nomically (empirically) possible, but these metaphysical
arguments don't normally turn on empirical possibility.  (It's
very likely that a zombie replica of me is nomically impossible,
but it's the mere logical possibility of such a thing that provides
an argument for dualism.)

-- 
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."