On 5/11/2015 12:14 AM, Bruce Kellett wrote:
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.
I think this obfuscates the point. One says yes to the doctor not because one's conscious
thought is a computation, but rather because the doctor proposes to replace part of your
brain with something that will perform ALL the computations that part of the brain could
do. It is not that consciousness is a computation, rather it is a class of computations
that will map all possible (not just actual) environmental inputs into outputs. And
that's why a recording is ruled out - whether it would be conscious or not; it is not
counterfactually adequate.
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
But a brain replacement can't be a mapping between one input and one output - which is
what a recording is. It has to a be a mapping between a large class of inputs and outputs.
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.
No, I think it's true because it's not counterfactually correct. Whether you call it a
computation or just and look-up table is, as Russell points out, a matter of intuition
about size. How many counterfactuals must it deal with? Whether the ultrafinitism is
true or not, our theory of the world and consciousness should not depend on there being
infinities. So within ultrafinitism all TM's can be replaced by lookup tables. Or looked
at the other way around, a sufficiently enormous lookup table is a computer.
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.
I agree.
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.
Primitive matter is a strawman. No one I've know, even Vic Stenger, has held that matter
is anything more than the ontology of one's theory of physics. Physicist make different
models and some have field ontologies, some have spacetime, some have particles. So
"matter" is just whatever the model says it is. If the world is made of computations then
we could call computations "matter".
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.
But the interesting thing about Bruno's theory is that it proposes a solution to the
mind-body problem by making both of them computations. Aside from the UD, this is not
particularly radical. If you had an intelligent/conscious AI within a virtual environment
then the consciousness of the AI would be relative to that environment and both of them
would be computations. Bruno proposes that the relation can be expressed in terms of what
the AI would "believe", i.e. able to prove, about the environment. I find this
interesting aside from arguments trying to defeat some imaginary "primitive
physicalists". The UD is interesting because it makes Tegmarks mathematical-universe
idea more specific, something you might be able to draw inferences from.
Brent
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