You wrote: "This is the part where you're going astray, actually. We have no reason to think that human beings are NOT Turing-computable. In other words, human beings almost certainly are Turing machines."
at this stage, you've just assumed the conclusion. you've just assumed what you want to prove.
"Therefore, consciousness -- whatever we mean when we say that -- is indeed possible for Turing machines."
having assumed that A is true, it is easy to prove that A is true. You haven't given an argument.
"To refute this proposition, you'd need to present evidence of a human being performing an operation that can't be done by a Turing machine."
It's not my job to refute the proposition. Currently, as far as I can tell, the question is open. If I did refute it, then my (and several other people's) conjecture would be proven. But if I don't refute it, that doesn't mean your proposition is true, it just means that it hasn't yet been proven false. Those are quite different things, you know.
Well, how about this: physics as we know it can be approximated arbitrarily closely by a computable algorithm (and possibly computed directly as well, although I'm less sure about that. Certainly all calculations we can do involving manipulation of symbols are computable). Physics as we know it also seems to be correct to extremely precise degrees anywhere apart from inside a black hole.
Brains are physical things. Now when we consider that thermal noise should have more of an influence than the slight inaccuracy in any computation, what are the chances a b... (read more)
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