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

Thinking Machines 125ity to add 1 to a number bestows the ability to generate an infinite set ofnumbers, the ability to embed a proposition inside another propositionbestows the ability to think an infinite number of thoughts.To get propositions-inside-propositions out of the network displayedin the preceding diagram, one could add a new layer of connections tothe top of the diagram, connecting the bank of units for the whole propositionto the role slot in some bigger proposition; the role might be somethinglike "event-observed." If we continue to add enough layers, wecould accommodate an entire multiply nested proposition by etching afull tree diagram for it in connectoplasm. But this solution is clumsy andraises suspicions. For every kind of recursive structure, there would haveto be a different network hard-wired in: one network for a person thinkingabout a proposition, another for a person thinking about a propositionabout a person thinking about a proposition, a third for a person communicatinga proposition about some person to another person, and so on.> In computer science and psycholinguistics, a more powerful and flexiblemechanism is used. Each simple structure (for a person, an action, aproposition, and so on) is represented in long-term memory once, and aprocessor shuttles its attention from one structure to another, storing the\ itinerary of visits in short-term memory to thread the propositionV» together. This dynamic processor, called a recursive transition network,is especially plausible for sentence understanding, because we hear andread words one at a time rather than inhaling an entire sentence at once.We also seem to chew our eomplex thoughts piece by piece rather swallowingor regurgitating them whole, and that suggests that the mind isequipped with a recursive proposition-cruncher for thoughts, not just forsentences. The psychologists Michael Jordan and Jeff Elman have builtnetworks whose output units send out connections that loop back into aset of short-term memory units, triggering a-new cycle of activation flow.That looping design provides a glimpse of how iterative information processingmight be implemented in neural networks, but it is not enoughto interpret or assemble structured propositions. More recently, therehave been attempts to combine a looping network with a propositionalnetwork to implement a kind of recursive transition network out ofpieces of connectoplasm. These attempts show that unless neural networksare specially assembled into a recursive processor, they cannothandle our recursive thoughts.