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

Thinking Machines 123of one proposition. A quantifier is a symbol that can express "There existsa particular x who . . ." and "For all x it is true that ..." A thought canthen be captured in a proposition built out of symbols for concepts,roles, quantifiers, and variables, all precisely ordered and bracketed.Compare, for example, "Every forty-five seconds {there exists an X [whogets injured]}" with "There exists an X {who every forty-five seconds[gets injured]}." Our mentalese must have machinery that does somethingsimilar. But so far, we have no hint as to how this can be done in anassociative network.Not only can a proposition be about an individual, it must be treatedas a kind of individual itself, and that gives rise to a new problem. Connectoplasmgets its power from superimposing patterns in a single set ofunits. Unfortunately, that can breed bizarre chimeras or make a networkfall between two stools. It is part of a pervasive bugaboo for connectoplasmcalled interference or cross-talk.Here are two examples. The psychologists Neal Cohen and MichaelMcCloskey trained a network to add two digits. They first trained it toadd 1 to the other numbers: when the inputs were "1" and "3," the networklearned to put out "4," and so on. Then they trained it to add 2 toany other number. Unfortunately, the add-2 problem sucked the connectionweights over to values that were optimal for adding 2, and becausethe network had no hardware set aside to anchoring the knowledge ofhow to add 1, it became amnesic for how to add 1! The effect is called"catastrophic forgetting" because it is unlike the mild forgetting of everydaylife. Another example comes from a network designed by McClellandand his collaborator Alan Kawamoto to assign meanings to ambiguoussentences. For example, A bat broke the window can mean either that abaseball bat was hurled at it or that a winged mammal flew through it.The network came up with the one interpretation that humans do notmake: a winged mammal broke the window using a baseball bat!As with any other tool, the features that make connectoplasm goodfor some things make it bad for other things. A network's ability to generalizecomes from its dense interconnectivity and its superposition ofinputs. But if you're a unit, it's not always so great to have thousands ofother units yammering in your ear and to be buffeted by wave after waveof inputs. Often different hunks of information should be packaged andstored separately, not blended. One way to do this is to give each propositionits own storage slot and address—once again showing that not allaspects of computer design can be dismissed as silicon curiosities. Com-