3. Reasoning: Goal Trees and Rule-Based Expert Systems

Reporting on a Goal Tree program

A Goal Tree program can answer questions about its own behavior by reporting steps up (why questions) or down (how questions) in the actions it takes.

Simon’s Ant

The complexity of a program’s behavior is a consequence of the environment, not the complexity of the program.

Complexity (behavior) = Max ( Complexity (Program) , Complexity (Environment) )

An ant’s apparently complex behavior stems from the obstacles in its environment to avoid on its way home.

Rule-Based Expert Systems

Rule-based “expert” systems are deductions systems, they can answer questions about their behaviors as they are a form of goal trees.

Forward-chaining rule-based expert systems work from characteristics to deduce a result by a set of rules. Backward-chaining rule-based expert systems verify a proposition is true by going back to the check its characteristics through the rules.

Heuristics of knowledge engineering

How to set the program’s rules:

  1. Deal with specific cases: know all details of each specific case, not vague, general ideas from people (potato chips, tomatoes vs “squishy”)
  2. Understand the vocabulary items that make two cases different (frozen vs canned)
  3. Analyze when the program breaks down to understand the missing rule.

2. Reasoning: Goal Trees and Problem Solving

Problem reduction

Take a complicate problem and transform it into a simpler problem.

Start with safe transformations, the ones you are sure will work in any case. Then apply heuristic transformations, the ones that could work.

The problem simplification schema, may create “and node“, where the problem forks in several sub problems and “or node” where the problem may be solved with either one or another transformation. The resulting schema is usually called a “problem reduction tree“, “and/or tree” or “goal tree“.

In an “or node”, it helps to understand the depth of functional composition¬†(number of transformations to be applied after and “or” options of the branch) and the simplicity of solving each options to complete the problem resolution.


Everything depends on the domain of the problem and the knowledge required to solve it. Knowledge about knowledge, meta-knowledge, is power to solve problems.

  1. Start by evaluating what kind of knowledge is involved.
  2. Understand how the knowledge is represented. Each category of knowledge has its own way of being represented.
  3. Know how the knowledge is used.
  4. Know how much knowledge is required to solve the problem.
  5. Know what exactly knowledge does to solve the problem.