X-Message-Number: 14933 Date: Thu, 16 Nov 2000 07:17:04 -0500 From: Thomas Donaldson <> Subject: once more on brains and computers HI everyone! Various individuals have complained about my posting. Basically they believe that Turing machines can imitate ...WHAT? One fundamental issue here is just how far Turing machines can go. (I do want to make it clear here that I have no objections to the possibility of making something which works like a human brain. What remains unknown is whether or not such a device fits the Turing machine picture at all; and yes, depending on the behavior of the hardware in such an imitation, it may have to be biological or close to biological because no other materials match the job that biological ones can do). First of all, it is NOT enough to simply say that connections generally are close by. The problem is that we deal with many neurons, and the connections of one such neuron aren't independent of those of the others. The neurons might grow new connections to neurons with which they did not at first connect. Changing only one neuron won't work. Second, no one complaining about what I said dealt with the even more fundamental problem that it looks as if we also grow new neurons throughout our brain... many mammals do this to a lesser degree, but apes and monkeys do it much more. Anyone who wishes is welcome to produce a proof that such a creature (device) follows all the requirements of a Turing machine. I said originally that I did not consider it obvious that it does... not the same as saying that it doesn't. Why isn't it obvious? Basically because by its nature our brain does not in any sense write on a single long tape. The different neurons with different connections go off and write on many tapes, connected as if they were a treelike graph. Or if you insist on a tape, that tape will have to have the ability to split into 2 tapes, etc etc. Moreover, one simple feature of our brains is not matched at all by ANY Turing machine: they work in real time, while a Turing machine is a single computer with a very long tape on which it works. If we were only proving math theorems (or any other purely theoretical act) then this would make no obvious difference. However our literal survival may depend on whether or not we finish some computation in a small fixed time. Time is IMPORTANT, and any attempt to understand how we work must first understand that point. The creation of new neurons, on top of all of this, means that such a system does not have a clear limit on the number of tapes it's following. (Yes, in practice there is a limit, but it is FUZZY, not a definite figure). So you're welcome to discuss whether or not such a machine can be truly matched by a Turing machine. But don't forget any of the features I have given here: new connections, new neurons, and the need to do things much faster than any single machine, even the brightest such machine. Best wishes and long long life for all, Thomas Donaldson Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=14933