10. A rule-based reasoning system for reasoning with uncertainty for practice on a computer, comprising: a knowledge base portion comprising a plurality of system rules each having a premise and a conclusion to be determined true upon finding the premise true, said premise comprising at least one clause including a variable to which a value can be assigned; a representation portion for representing to a system user a value certainty interval associated with each value assigned to one of said rule premise variables, said value certainty interval having a lower and an upper bound respectively representative of amounts of confirmation and failure of refutation of the assigned value; said representation portion additionally representing to the user a sufficiency factor and a necessity factor associated with each said rule, said sufficiency factor representing a strength of belief that said rule conclusion is true given said rule premise is true, said necessity factor representing a strength of belief that said rule conclusion is false given said rule premise is false; a control portion for selecting computational functions in accordance with which the value certainty interval associated with each said assigned value is propagated through said rule based system; an inference portion for computing a premise certainty interval from the value certainty intervals respectively associated with the values assigned to the premise clauses, a lower bound of said premise certainty interval being a first computational function of the respective value certainty interval lower bounds, said first function being selected in said control portion, an upper bound of said premise certainty interval being said first function of the respective value certainty interval upper bounds; and said inference portion firing each said rule by computing a conclusion detachment interval having a lower and an upper bound, the conclusion detachment interval lower bound being a second computational function of both said rule sufficiency factor and said premise certainty interval lower bound, the conclusion detachment interval upper bound being a third computational function of both said rule necessity factor and said premise certainty interval upper bound, said second and third functions being selected by said control portion, said conclusion detachment interval lower and upper bounds being respectively representative of the amounts of confirmation and failure of refutation of the rule conclusion.
10. A rule-based reasoning system for reasoning with uncertainty for practice on a computer, comprising: a knowledge base portion comprising a plurality of system rules each having a premise and a conclusion to be determined true upon finding the premise true, said premise comprising at least one clause including a variable to which a value can be assigned; a representation portion for representing to a system user a value certainty interval associated with each value assigned to one of said rule premise variables, said value certainty interval having a lower and an upper bound respectively representative of amounts of confirmation and failure of refutation of the assigned value; said representation portion additionally representing to the user a sufficiency factor and a necessity factor associated with each said rule, said sufficiency factor representing a strength of belief that said rule conclusion is true given said rule premise is true, said necessity factor representing a strength of belief that said rule conclusion is false given said rule premise is false; a control portion for selecting computational functions in accordance with which the value certainty interval associated with each said assigned value is propagated through said rule based system; an inference portion for computing a premise certainty interval from the value certainty intervals respectively associated with the values assigned to the premise clauses, a lower bound of said premise certainty interval being a first computational function of the respective value certainty interval lower bounds, said first function being selected in said control portion, an upper bound of said premise certainty interval being said first function of the respective value certainty interval upper bounds; and said inference portion firing each said rule by computing a conclusion detachment interval having a lower and an upper bound, the conclusion detachment interval lower bound being a second computational function of both said rule sufficiency factor and said premise certainty interval lower bound, the conclusion detachment interval upper bound being a third computational function of both said rule necessity factor and said premise certainty interval upper bound, said second and third functions being selected by said control portion, said conclusion detachment interval lower and upper bounds being respectively representative of the amounts of confirmation and failure of refutation of the rule conclusion. 19. The reasoning with uncertainty system of claim 10 wherein selected ones of said rules are each associated with a respective source of rule firing context information; and said control portion evaluating, prior to the firing of each said selected rule, said context information to determine whether an appropriate context exists for the rule firing.