Bruno Marchal wrote:
On 07 May 2015, at 14:45, Bruce Kellett wrote:
......
Now, having read this many times, and looked at the other summaries of
the MGA, I still feel that something crucial is missing. We go from
the situation where we remove more and more of the original 'brain',
replacing the removed functionality by the projections from the movie,
which, it is agreed, does not alter the conscious experience of the
first person involved, to the conclusion that the physical brain is
entirely unnecessary; indeed, irrelevant.
Hmm... On the contrary: the brain is necessary. It is the primitive
physicalness of the brain which is not relevant.
That is not what you say in the paper. "Hence, consciousness is not a
physical phenomenon, nor can it be a phenomenon relating to observed
matter at all." You go on to say that the appearance of matter cannot be
based on a notion of primitive matter. But these are different things.
Elsewhere you appear to agree that consciousness does depend on the
observed physical brain. In fact, it would be foolish to deny this given
the weight of physical evidence that shows this to be the case.
Now that I have had a couple of days away from the internet to think
about this, and have read other comments on this thread, I think I
understand better the point that was not clear to me from the COMP(2013)
paper. What your intuition claims to be absurd in the MGA is that
replaying the film can instantiate consciousness. The reason for this is
based on your belief that replaying the film is not a computation, and
since the basic assumption is of comp is that consciousness is Turing
emulable -- is in fact a computation -- we cannot have consciousness
without the associated computation.
The argument is then that if the assumption of physical supervenience
(supervenience of consciousness on a physical brain) leads to a
situation in which consciousness would appear to be supported by
something (the film) which is not a computation, then a contradiction
has been reached, and the idea of physical supervenience must be wrong
(if comp is correct).
That makes sense, but I did not previously accept this because my
intuition was not that the projection of the film would not reconstitute
the original conscious moment. The important point that is now clear, is
that you claim that projection the film does not constitute a
computation, so cannot support consciousness. I disagree with this. As
Russell has suggested, projecting the film can very well be considered
to be a computation.
We have to ask what constitutes a computation in the context of this
discussion. The starting point is that part or all of the brain is
replaceable by a computer -- the brain is Turing emulable. So it seems
reasonable to define a computation as a mapping between some input and
some output that is Turing emulable. In other words, one can replace the
device that takes some input to produce some particular output with a
general Turing machine. That mapping from input to output would then be
considered a computation in the terms of the present discussion of the
comp thesis.
Defined in this way, it is clear that projecting the movie film on to
the physical substrate is nothing more than a general computation. The
input is a source of light directed on to the film, and the output is
the image focussed on the screen (or brain substrate). If you like, to
use Russell's terms again, the film is a program that is run through the
projector as a computer. This process is completely emulable by a Turing
machine. In fact, digital projections of moving images are routinely
performed on general purpose digital computers. The film (program) can
be stored digitally, and the light source and screen can also be
realized digitally.
The claim that the film (and projection) is not a computation is thus
false. It is a computation in exactly the same way that the brain
function replaced by a Turing machine in the "yes doctor" step 0 of the
argument is a computation. So the MGA does not establish the conclusion
that "consciousness can no longer be related to any physical
phenomenon whatsoever (i.e., brains in skulls), nor can any subjective
appearance of matter be based on a notion of primitive matter."
In fact, the MGA seems to have very little to do directly with the
hypothesis of primitive physicality. The argument appears to be that if
physical supervenience (a different notion than primitive physicality)
leads to a contradiction with the comp hypothesis, then physical
supervenience must be abandoned. Extending this line of thinking, it
appears to be suggested that if physical supervenience is abandoned,
there is no remaining role for primitive physical matter in the
understanding of consciousness. The argument is less clear at this
point, but something of the sort seems to be implied.
But if the notion of physical supervenience cannot be ruled out, then
the way is open for primitive physicality. The comp argument, which
claims that the appearance of the physical can be extracted from the UD
running in Platonia, has no greater claim to credence than the
physicalist's claim that mathematics is a human invention, extracted
from our experience of the physical world.
The choice between these might reduce to nothing more than personal
preference.
Bruce
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