backwardchaining

Backward chaining is a goal-driven inference technique used in rule-based expert systems and logic programming. Unlike forward chaining, which starts from known data and derives new facts, backward chaining begins with a hypothesis or query and works backward to determine whether it can be supported by the available rules and facts.

Mechanism: given rules of the form If A and B then C, to prove C the system attempts to prove A and B. Each subgoal is recursively expanded using rules whose conclusions match the subgoal, or by consulting known facts. The procedure continues until the goal is proven or no support remains. If a subgoal matches a fact, it is considered proven; if multiple rules could derive a subgoal, branches are explored in search of a proof. Cycles must be avoided to prevent infinite loops.

Implementation: Prolog is a well-known language that employs backward chaining in evaluating queries against a database of Horn clauses. Backward chaining is particularly suitable for situations where the set of possible goals is limited or where the cost of proving individual goals is high but their subgoals are manageable.

Advantages and limitations: It is efficient when the goal is specific and the rule base is large, since only relevant rules are considered. However, it can suffer from nontermination in the presence of recursive or cyclic rules, and in some cases may explore many irrelevant subgoals if not well-pruned. The approach is common in diagnostic reasoning, planning, and troubleshooting domains.