Tautology: A tautology is a proposition that is always ture.
ex: (p v ~p)= T
Contradiction: A contradiction is a proposition that is always false.
ex: (p ^ ~p)=F
Contigency: A contigency is a proposition that is neither a tautology nor a contradiction.
ex: (p v q)----> ~r
* A propositional logic is said to be satisfiable if it is either a tautology or contigency.
* If logic is contradiction then it is said to be unsatisfiable.
* By contigency we means the logic can be ture or false.
* Contradiction is the unsatisfiable function.
* Statement is valid means tautology.
* Statement is not valid means not tautology.