- Knowledge base is a set of sentences.
- proposition letters p, q, r ... they generate set of different situations. eg. three propositional letters generate 2^3 = 8 situations
- propositions are all true. ?? 不对。
- Tautology is a statement that is always true.
- A formula is satisfiable if there exists an assignment of truth values which makes the formula true.
- An inference is valid if under any assignment of values which make premises true, conclusion is also true.