Despite these successes, the symbolic approach has proven insufficient. It has not adequately modeled intelligent behavior such as scene analysis or comprehensive speech recognition. Morever, even the so-called successful applications could not achieve a desirable characteristic known as graceful degradation, which is the ability to know when the limits of knowledge are reached and thereby to give up or fall hack on “common sense” in an attempt to fill in gaps. For example, humans successfully cope with ungrammatical and incomplete utterances. Machines usually cannot.
Common sense would seem to require far more symbols than the human brain can hold and process in real-time (half a second). Experimental evidence from cognitive psychology and neurophysiology indicates that the mind cannot possibly process a “program” of more than about a hundred steps in this time frame. What’s more, in order to respond as humans do, symbolic programs would have to execute thousands of ...
