Chaining expert system logic

Each rule in an expert system knowledge base follows a conditional logic approach to reasoning. If a condition is satisfied, then the system makes a deduction. The rules in an expert system’s knowledge base can be connected or chained together. This chaining allows the system to make several deductions in sequence.


To make this forward-chaining approach work, the expert system needs to enter multiple facts or conditions into the system. The rules are applied to these conditions to the chain of rules.

Say the rules in a knowledge base look like this:anchainedThen say the following facts are added:

  1. we have an unknown subject
  2. the unknown subject is human made
  3. the human made subject is a vehicle
  4. the vehicle flies
  5. the flying vehicle has fixed wings

Now apply these facts to the rules to allow the expert system to apply forward chaining logic.


This forward chaining, based on the multiple facts entered into the system, concluded that the subject is a fixed wing aircraft. The expert system used a series of conditional logic statements to make that deduction:

  • if the subject is human made, and
  • if the subject is a vehicle, and
  • if the vehicle flies, and
  • if the flying vehicle has fixed wings,

then the subject is a fixed wing aircraft.

Chaining can also move backward. Using the earlier example, we can use conditional logic to deduce:

If the subject is a fixed wing aircraft, then:

  • it has fixed wings, and
  • it flies, and
  • it is a vehicle, and
  • it is human made



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