On 15/06/2016 2:11 pm, Brent Meeker wrote:
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.
Just pointing to a conscious person does not specify what consciousness
is, or its limits. Much less does it indicate that consciousness is a
kind of computation.
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.
That seems to be assuming a lot! Assuming that consciousness is a (type
of) computation does not imply that non-arithmetical substrates exist,
much less that pencil and paper exist. Knowing that something is true of
the world that we experience does not entail that its existence is
necessary.
Bruno can start from his (neo-)platonist assumption that arithmetic
exists independently, and that arithmetic implements all computations
(Turing machines). But he then has to prove that this assumption leads
necessarily to the existence of a physical world of the character that
we observe. Since his 7 steps only work if the physical world is already
assumed, he has to look to some other arguments. I don't think you can
argue that the physical world is assumed in Bruno's 7 steps as part of a
reductio argument -- that this assumption leads to a contradiction. In
the first place, the reductio fails, and even if it succeeded, one could
well claim the assumption of platonism was the root of the
contradiction, leaving physicalism unscathed.
Bruce
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