6. A knowledge system comprising a computer having a memory, said memory storing a knowledge base including expressions and knowledge for determining values for said expressions, said memory including a cache porton for storing certain ones of said expressions and corresponding determined values for the stored expressions, said memory also storing a control procedure executable by said computer for interpreting the knowledge base to determine values for said expressions, said control procedure including means for recognizing the occurrence of an expression in the knowledge base, means for searching the cache to determine whether the recognized expression is stored in the cache, means for obtaining from the cache the corresponding value of the recognized expression when the recognized expession is in the cache, and means for applying the knowledge in the knowledge base to determine a corresponding value for the recognized expression when the recognized expression is not in the cache and then storing the recognized expression and its corresponding value in the cache.
8. The knowledge system as claimed in claim 6, wherein said cache includes memory space for storing certainty factors corresponding to the value of said certain ones of said expressions, and said control procedure includes means for interpreting the knowledge base to determine a certainty factor for the value determined for the recognized expression when the recognized expression is not in the cache and then storing the determined certainty factor in the cache along with its corresponding expression and value.