How do IF-THEN rules model problem solving and planning?

In the context of cognitive science, rule-based models represent thinking as a process of applying inference rules structured as "IF...THEN..." statements to complex symbols. These symbols typically possess an internal structure similar to natural language sentences.1

When modeling problem solving and planning, these systems function through a step-by-step sequence of inferences. An initial symbol is processed against a rule to yield a new symbol; this resulting symbol is then used as an input for subsequent rules, creating a chain of logical deductions. For example, if a system holds the rule "IF you have a stomach ache, THEN you should lie down," it can take the output of a previous rule—such as "you have a stomach ache"—and apply it to reach the conclusion "you should lie down." By chaining these inferences together, rule-based systems can model complex cognitive tasks and have been instrumental in the development of expert systems for fields like medicine.1