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Rick Grush & Patricia Smith Churchland
University of California at San Diego
La Jolla, CA 92093
``Using the Gödel incompleteness result for leverage, Roger Penrose has argued that the mechanism for consciousness involves quantum gravitational phenomena, acting through microtubules in neurons. We show that this hypothesis is implausible. First the Gödel result does not imply that human thought is in fact non-algorithmic. Second, whether or not non-algorithmic quantum gravitational phenomena actually exist, and if they did how that could conceivably implicate microtubules, and if microtubules were involved, how that could conceivably implicate consciousness, is entirely speculative. Third, cytoplasmic ions such as calcium and sodium are almost certainly present in the microtubule pore, barring the quantum-mechanical effects Penrose envisages. Finally, physiological evidence indicates that consciousness does not directly depend on microtubule properties in any case, rendering doubtful any theory according to which consciousness is generated in the microtubules.''
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University of Oxford
24-29 St. Giles
Oxford OX1 3LB
Department of Anesthesiology
University of Arizona Health Sciences Center
Tucson, AZ 85724
Grush and Churchland attempt to address aspects of the proposal that we have been making concerning a possible physical mechanism underlying the phenomenon of consciousness. Unfortunately, they employ arguments that are highly misleading and, in some important respects, factually incorrect. Their article ``Gaps in Penrose's Toilings'' is addressed specifically at the writings of one of us (Penrose), but since the particular model they attack is one put forward by both of us (Hameroff and Penrose, 1995; 1996), it is appropriate that we both reply; but since our individual remarks refer to different aspects of their criticism we are commenting on their article separately. The logical arguments discussed by Grush and Churchland, and the related physics are answered in Part 1 by Penrose, largely by pointing out precisely where these arguments have already been treated in detail in Shadows of the Mind (Penrose, 1994). In Part 2 Hameroff replies to various points on the biological side, showing for example how they have seriously misunderstood what they refer to as ``physiological evidence'' regarding effects of the drug colchicine. The reply serves also to discuss aspects of our model ``orchestrated objective reduction in brain microtubules - Orch OR'' which attempts to deal with the serious problems of consciousness more directly and completely than any previous theory.
This debate is being continued on the Psyche-D and JCS-online discussion lists.
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