The inference engine enables the expert system to draw deductions from the rules in the knowledge base (KB). e.g., if the KB contains the production rules "if x, then y" & "if y, then z," the inference engine is able to deduce "if x, then z." The expert system might then query its user, "Is x true in the situation that we are considering?" If the answer is affirmative, the system will proceed to infer z.