Negation, also known as a logical NOT, is a basic operation in propositional logic. The statment (-. ph) is true when the statement ph is not true.
The properties of negation are derived directly from the logical axioms. Only one of these axioms involves negation. The principle of transposition, also call contraposition (-> (-> (-. ph) (-. ps)) (-> ps ph)) /edit/peano_new/prop_min.ghi/ax-3.