2. A knowledge engineering tool comprising a computer having memory for storing a knowledge base and memory for storing predetermined instructions including means to augment and edit the knowledge base, and means for interpreting the knowledge base to run an interactive consultation with the user to determine the value of at least one predetermined goal expression, wherein said knowledge base includes rules including premises having logical operations and corresponding conclusions concluding at least one value for said goal expression, said rules including rules having universally quantified variables, certainty factors associated with said rules for encoding uncertain as well as certain knowledge, propositions including arithmetic operations, propositions responsive to whether a value of a predetermined expression has been determined with a predetermined degree of certainty, and wherein said means for interpreting the knowledge base includes means for determining the value of said goal expression including means for invoking and chaining said rules to conclude a value for said goal expression, means for evaluating the logical operations of the premises of the invoked rules and selecting certainty factors for the concluded values based on the certainty factors of the rules and the certainty factors of the premises, means for updating the certainty factor of a value of an expression by combining certainty factors, means for evaluating said propositions, means for instantiating and deinstantiating the universally quantified variables in said rules, and means for conveying to the user said value of said goal expression and its certainty factor.
2. A knowledge engineering tool comprising a computer having memory for storing a knowledge base and memory for storing predetermined instructions including means to augment and edit the knowledge base, and means for interpreting the knowledge base to run an interactive consultation with the user to determine the value of at least one predetermined goal expression, wherein said knowledge base includes rules including premises having logical operations and corresponding conclusions concluding at least one value for said goal expression, said rules including rules having universally quantified variables, certainty factors associated with said rules for encoding uncertain as well as certain knowledge, propositions including arithmetic operations, propositions responsive to whether a value of a predetermined expression has been determined with a predetermined degree of certainty, and wherein said means for interpreting the knowledge base includes means for determining the value of said goal expression including means for invoking and chaining said rules to conclude a value for said goal expression, means for evaluating the logical operations of the premises of the invoked rules and selecting certainty factors for the concluded values based on the certainty factors of the rules and the certainty factors of the premises, means for updating the certainty factor of a value of an expression by combining certainty factors, means for evaluating said propositions, means for instantiating and deinstantiating the universally quantified variables in said rules, and means for conveying to the user said value of said goal expression and its certainty factor. 4. The knowledge engineering tool as claimed in claim 2, wherein at least one of said universally quantified variables is used in both the premise and conclusion of one of said rules, and said means for instantiating and deinstantiating instantiates the variable in the premise of the rule thereby enabling the value of the instantiated variable to be used in asserting the conclusion when the rule is applied.