Steven Pinker -- How the Mind Works - Hampshire High Italian ...

Thinking Machines 99"Whatever Logic is good enough to tell me is worth writing down. Soenter it in your book, please. We will call it(E) If A and B and C and D are true, Z must be true.""I see," said Achilles; and there was a touch of sadness in his tone.Here the narrator, having pressing business at the Bank, was obligedto leave the happy pair, and did not again pass the spot until somemonths afterwards. When he did so, Achilles was still seated on the backof the much-enduring tortoise, and was writing in his notebook, whichappeared to be nearly full. The tortoise was saying, "Have you got thatlast step written down? Unless I've lost count, that makes a thousand andone. There are several millions more to come."The solution to the paradox, of course, is that no inference system followsexplicit rules all the way down. At some point the system must, asJerry Rubin (and later the Nike Corporation) said, just do it. That is, therule must simply be executed by the reflexive, brute-force operation ofthe system, no more questions asked. At that point the system, if implementedas a machine, would not be following rules but obeying the lawsof physics. Similarly, if representations are read and written by demons(rules for replacing symbols with symbols), and the demons have smaller(and stupider) demons inside them, eventually you have to call Ghostbustersand replace the smallest and stupidest demons with machines—in the case of people and animals, machines built from neurons: neuralnetworks. Let's see how our picture of how the mind works can begrounded in simple ideas of how the brain works.The first hints came from the mathematicians Warren McCullochand Walter Pitts, who wrote about the "neuro-logical" properties of connectedneurons. Neurons are complicated and still not understood, butMcCulloch and Pitts and most neural-network modelers since haveidentified one thing neurons do as the most significant thing. Neurons,in effect, add up a set of quantities, compare the sum to a threshold, andindicate whether the threshold is exceeded. That is a conceptualdescription of what they do; the corresponding physical description isthat a firing neuron is active to varying degrees, and its activity level isinfluenced by the activity levels of the incoming axons from other neuronsattached at synapses to the neuron's dendrites (input structures). Asynapse has a strength ranging from positive (excitatory) through zero (noeffect) to negative (inhibitory). The activation level of each incomingaxon is multiplied by the strength of the synapse. The neuron sums these