On the standard textbook definition of computation, it is hard to see how to avoid the following results:
For any object there is some description of that object such that under that description the object is a digital computer.
For any program and for any sufficiently complex
object, there is some description of the object under which
it is implementing the program. Thus for example the wall
behind my back is right now implementing the Wordstar
program, because there is some pattern of molecule movements that is isomorphic with the formal structure of
Wordstar. But if the wall is implementing Wordstar, then
if it is a big enough wall it is implementing any program,
including any program implemented in the brain. [...]
I do not think that the problem of universal realizability is a
serious one. I think it is possible to block the result of universal
realizability by tightening up our definition of computation.
Certainly we ought to respect the fact that programmers and
engineers regard it as a quirk of Turing’s original definitions
and not as a real feature of computation. Unpublished works
by Brian Smith, Vinod Goel, and John Batali all suggest that a
more realistic definition of computation will emphasize such
features as the causal relations among program states, programmability
and controllability of the mechanism, and situatedness
in the real world. All these will produce the result that
the pattern is not enough. There must be a causal structure
sufficient to warrant counterfactuals. But these further restrictions
on the definition of computation are no help in the
present discussion because the really deep problem is that syntax is
essentially an observer-relative notion. The multiple realizability of
computationally equivalent processes in different physical media is
not just a sign that the processes are abstract, but that they are not
intrinsic to the system at all. They depend on an interpretation from
outside. We were looking for some facts of the matter that
would make brain processes computational; but given the way
we have defined computation, there never could be any such
facts of the matter. We can’t, on the one hand, say that anything
is a digital computer if we can assign a syntax to it, and
then suppose there is a factual question intrinsic to its physical
operation whether or not a natural system such as the brain is
a digital computer.
And if the word “syntax” seems puzzling, the same point
can be stated without it. That is, someone might claim that the
notions of “syntax” and “symbols” are just a manner of speaking
and that what we are really interested in is the existence of
systems with discrete physical phenomena and state transitions
between them. On this view, we don’t really need 0′s
and l’s; they are just a convenient shorthand. But, I believe,
this move is no help. A physical state of a system is a computational
state only relative to the assignment to that state of
some computational role, function, or interpretation. The
same problem arises without 0′s and l’s because notions such as
computation, algorithm, and program do not name intrinsic physical
features of systems. Computational states are not discovered
within the physics, they are assigned to the physics.
On page 208-210 of The Rediscovery of the Mind, Searle writes: