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>From: chalmers@bronze.ucs.indiana.edu (David Chalmers)
Subject: Re: Causes and Reasons
Message-ID: <1992Jan12.211951.20547@bronze.ucs.indiana.edu>
Organization: Indiana University
References: <5918@skye.ed.ac.uk> <1992Jan10.011118.26218@bronze.ucs.indiana.edu> <5943@skye.ed.ac.uk>
Date: Sun, 12 Jan 92 21:19:51 GMT
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In article <5943@skye.ed.ac.uk> jeff@aiai.UUCP (Jeff Dalton) writes:

>This sounds plausible, but you still seem to be thinking of the
>program as a fairly direct description of what will happen at the
>hardware level on particular kinds of machines.  Your neural net
>program could be transformed into a program with the same I/O
>behavior but in which there wasn't a variable for the activation
>of each unit.

This seems to be the key point.  The notion of "implementing an
algorithm" that I'm using is stronger than mere I/O equivalence, but
rather puts certain restrictions on states and state-transitions --
so that e.g. there should be identifiable physical states determining
the value of a given variable, and so on.  So insofar as certain
compilers are only concerned with preserving mere I/O equivalence,
then they're not implementing the program in the sense that I'm using.
Otherwise you reach conclusions such as that one could implement an
outputless 5000 line program statements with a trivial process that
ignores the complexity of the original program; or one could implement
a neural network with a lookup table.  I'm concerned here with Marr's
algorithmic level, not his "computational" level (a terrible choice of
word, incidentally).

Incidentally I recall that there was a Ph.D. thesis that came out of
Edinburgh a year or two ago on just this topic, giving a good account
of implementation and algorithmic equivalence.

-- 
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."


