Truth Values
Every statement evaluates to true, false, or unknown (see
World Assumption). How conditions combine:
| Combination | True | False | Unknown |
|---|---|---|---|
| Required conditions with AND | Every condition is true | At least one condition is false | No false condition, at least one unknown |
| Alternatives with OR | At least one alternative is true | Every alternative is false in a complete candidate set | No true alternative, at least one unknown |
Two consequences: unknown evidence is preserved unless a false condition
already decides the result, and deleting a negative fact produces unknown,
never true.