(Reply to Bernard Baars)
I don’t think anyone on any side of this discussion has said that the brain is a Turing Machine. The one who comes closest, Shimon Edelman, explicitly says “I argue that feelings in fact are computations, albeit not Turing computations.”
A Turing robot (i.e., a robot capable of passing the Turing Test, indistinguishably from any of the rest of us, for a lifetime) is not a computer (Turing machine). It is a dynamical system, with sensors and effectors, and on the inside it may be implementing any processes — whether dynamic or computational — that give it the capacity to pass the Turing Test, Turing computation being only one among the many possible processes.
The “weak” version of the Church-Turing Thesis is that everything that is “effectively computable” for a mathematician is computable by a Turing Machine.
The strong version of the Church-Turing Thesis is that Turing computation (digital computation) can simulate and approximate (just about) any dynamical physical process in the universe, including sensors and effectors, as well as analog continuous, parallel, distributed processes (such as internal rotation), and indeed also just about any neuro-chemical brain processes (perhaps excluding quantum and chaotic processes). But that simulation is only formal. A purely computational airplane does not fly. And a purely computational brain does not cognize (nor, a fortiori, does it feel). Nor does a purely computational robot (a “virtual robot”).
It is an empirical question, however, what and how much of the actual internal functioning of a Turing robot (or brain) could be performed by Turing computation.
What’s sure is that it cannot be all of it.
BB: “I realize that traditionally Turing Machines are taken to be abstract versions of all possible computational implementations, including bio computation. If you can therefore prove, or quasi-prove, that something is possible or impossible for a Turing Machine that is taken to apply to all possible computers. The trouble is that the assumption is wrong.“
The strong version of the Church-Turing Thesis holds that Turing computation can simulate and approximate (just about) any dynamical physical process — not that it can stand in for any dynamical physical process. You can’t fly to Chicago on a simulated airplane; flying is not computation. But computation can decompose and test the causal explanation of flying (or cognition).
BB: “1. Turing Machines have no memory, and no time, and no string limits. Those are non-biological assumptions.“
Turing machines are formal abstractions, but they can be implemented in real finite-state dynamical systems, for example, digital computers (which do have memories, clocks and length limits).
BB: “2. Turing Machines are rigidly serial, when the brain is a massively parallel, and parallel-interactive organ.“
Yes, but as noted, nobody says the brain is a Turing machine, just that the brain can be simulated computationally by a Turing machine.
BB: “3. While it is argued that TM’s can simulate parallel and parallel-interactive computations, that is plausible only because TM’s totally ignore memory, time, and finite string limits.“
They can simulate them because the parallelism is simulated serially, in virtual rather than real time.
BB: “4. I believe that Stan Franklin and a colleague have given a formal proof that contrary to earlier claims, there are formal machines that are more powerful mathematically than Turing Machines. This vitiates the whole standard use of TMs.“
The subject of hypercomputation is controversial and I think the “hard” problem of explaining feeling is hard enough without complicating it with speculations about hypercomputation (or quantum mechanics!).
The weak Church-Turing Thesis stands unrefuted to date: Whatever mathematicians have regarded as computation has turned out to be Turing machine-computable.
The strong Church-Turing Thesis does not hold that everything is computer-simulable, only just-about everything.
BB: “5. Consciousness and qualia are biological entities, which are selectionist rather than instructionist in principle (GM Edelman), and reflect a huge evolutionary history — 200 million for mammals alone.“
No doubt. But feeling (i.e., consciousness, qualia) poses a special, hard hard problem, both for evolutionary explanation and for functional/causal explanation. This problem will be the subject of the 2012 Summer School on the Evolution and Function of Consciousness at the Université du Québec à Montreal in June/July 2012 in which many of the contributors to this discussion (including Bernie Baars) and many other thinkers will be participating. (The Summer School will also be in commemoration of the centennial of Turing’s birth in June 1912).
BB: “6. We have a long and repeated history of ‘impossibility proofs” designed to falsify important empirical advances. Newton’s action at a distance, the molecular basis of life, etc. These efforts routinely fail, though they sometimes do so in interesting ways.“
Explaining how and why we feel is hard (indeed, I think, impossible), but the reason has nothing to do with Turing machines or computation, nor with either the weak or the strong Church-Turing Thesis. (See “Vitalism, Animism and Feeling (Reply to Anil Seth)” in this discussion.)
BB: “7. There is no substitute for looking at nature.“
Logic is an ineluctable part of nature too…
Harnad, S. (2008) The Annotation Game: On Turing (1950) on Computing, Machinery and Intelligence. In: Epstein, Robert & Peters, Grace (Eds.) Parsing the Turing Test: Philosophical and Methodological Issues in the Quest for the Thinking Computer. Springer