Abstract: An apparatus and method for an expert knowledge system developmental tool 50 comprising a computer 52 with a structure for storing a knowledge base 54 and an inference engine 56 for inferring from the knowledge base 54. The knowledge base 54 defines a plurality of positive and negative rules 402,410, each rule having one or more antecedents 302 and one conclusion 328. The interference engine 56 includes structure 100 for storing a hypothesis list and structure 100 for selecting rules that have a conclusion which match each hypothesis and the negative of each hypothesis. Further, the inference engine 56 includes structure 116-150 for analyzing the selected rules 402, 410 with respect to each other in order to determine a value of each hypothesis, and structure 162-184 for analyzing the antecedent 302 of each rule to determine a value for each rule 402, 410.