Expert Systems Reasonable Reasoning

Facts are not facts

• Morphology is rod requires microscopic evaluation.
• Is a bean shape a rod?
• Is an “S” shape a rod?
• Morphology is rod is assigned a confidence.
• All “facts” assigned confidences, from 0 to 1.

MYCIN: Shortliffe

• Begins with a few facts about patient
• Required by physicians but irrelevant
• Backward chains from each possible goal (disease).
• Preconditions either match facts or set up new subgoals. Subgoals may involve tests.
• Finds all “proofs” and weighs them.
• Explains decisions and combines evidence
• Worked better than average physician.
• Never used in practice.
• Methodology used.

Examples and non-examples

• Soybean diagnosis
• Expert codified knowledge in form of rules
• System almost as good
• When hundreds of rules, system seems reasonable.
• Autoclade placement
• Expert but no codification
• Chess
• Experts but no codification in terms of rules

Forward Chaining Interpreter

• Repeat
• Apply all the rules to the current facts.
• Each rule firing may add new facts
• Until no new facts are added.
• Comprehensible
• Trace of rule applications that lead to conclusion is explanation. Answers why.

Forward Chaining Example

• Facts:
• F1: Ungee gives milk
• F2: Ungee eats meat
• F3: Ungee has hoofs
• Rules:
• R1: If X gives milk, then it is a mammal
• R2: If X is a mammal and eats meat, then carnivore.
• R3: If X is a carnivore and has hoofs, then ungulate
• Easy to see: Ungee is ungulate.

Backward Chaining

• Start with Goal G1: is Ungee an ungulate?
• G1 matches conclusion of R3
• Sets up premises as subgoals
• G2: Ungee is carnivore
• G3: Ungee has hoofs
• G3 matches fact F3 so true.
• G2 matches conclusion of R2. etc.