On 6/14/2016 7:18 PM, Bruce Kellett wrote:
On 15/06/2016 11:55 am, Brent Meeker wrote:
On 6/14/2016 4:56 PM, Bruce Kellett wrote:
On 15/06/2016 5:22 am, Brent Meeker wrote:
On 6/14/2016 10:09 AM, Bruno Marchal wrote:
Let me explain shortly. First we start from consciousness, by
(re)defining computationalism as the assumption that there is a
level of description of myself such that my consciousness remains
unchanged through a functional substitution made at that level.
But already at the beginning you have swept the problem under the
rug. Notice that you could replace "consciousness" by "physics" in
the same sentence. You're just assuming that whatever you're
talking about can be computed - which is OK, but it's not solution
to the problem of consciousness until you can say exactly which
computations are conscious an which are not. I think it is
interesting that you consider spiders conscious, but not plants.
What's the difference? Obviously it's the degree and scope of
interaction with the environment. Which to me is further evidence
that you implicitly recognize there can be no sharp division
between matter and mind.
I agree with you here, but I think that Bruno has an even more
serious problem: it seems that there is an inherent circularity in
the above computationalist account of consciousness.
The starting assumption is that consciousness is unchanged by a
functional substitution at some level. But what does a "functional
substitution" mean in this context? It is clear that Bruno is
thinking of replacing some or all of the human brain by a
functionally identical machine. Firstly, that assumes supervenience
of consciousness on the brain -- something that is not part of the
definition of consciousness.
But one for which there is good evidence.
Sure, but is that part of the definition of consciousness?
I don't think he ever intended to define consciousness. He assumes
everyone knows what it is, i.e. ostensive definition.
And secondly, it assumes that a different substrate, one that can
instantiate computations independently of brains and consciousness,
exits.
Which follows from the Church-Turing thesis that all Turing universal
computers can compute the same set of functions.
No, the existence of an independent substrate does not follow from the
Church-Turing thesis. That thesis merely states that *if* you can
implement a Turing machine on a different substrate, it will be able
to compute the same functions. That does not require that any such
substrate exists.
But we already know that substrates exist that will support a universal
Turing machine (modulo infinite memory tape), i.e. digital computers.
Turing imagined his machine to be implemented by pencil and paper and a
set of instructions.
Brent
If you are going to substitute something for something else, you
need something else by which to make the substitution. In this case,
the implicit assumption is that we have a physical computer that can
be used to carry out the required computations. But no such physical
machines exist if we start with consciousness in isolation.
That's not how I understand his argument. He does start from the
assumption that a brain's consciousness can be instantiated by any
machine that is Turing universal - what he calls the "Yes, doctor"
assumption. But then he tries to prove that the physical
instantiation is superfluous by constructing a thought experiment in
which the computation takes place with no corresponding physical
changes of state (see also Mauldin's Olympia argument). I don't think
he succeeds in this proof, but I don't think he considers it very
important either. As a neo-platonist he's already sure that numbers,
arithmetic, and computations are "more real" than material objects.
So the immaterial existence of computations is almost a given for Bruno.
I don't think that Bruno's MGA (or Maudlin's Olympia argument) succeed
either. So it has not been demonstrated that consciousness can exist
without a physical substrate. Neo-platonism is a philosophical stance
(and not a dominant one among philosophers or scientists), and the
independent existence of numbers and arithmetic in a non-physical
'platonic' realm is not a law of thought. It is perfectly rational to
deny this and to assume some version of physicalism. In fact, in a
survey of philosophers by Bourget and Chalmers (Philosophical Studies
3:1-36) Physicalism is the dominant philosophy of mind (57%).
Bruce
Bruno wants to deduce the existence of the physical by some
statistics over computations going through the particular
consciousness. But this is viciously circular if he has to assume
the existence of that physical level at the start. He hasn't derived
or deduced it -- he has simply assumed it.
Bruce
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