 Our next clauses are for the negation sign, “\(\neg\)”. The underlying idea is that a sentence \(\psi\) is inconsistent with its negation \(\neg \psi\). They cannot both be true. We call a pair of sentences \(\psi, \neg \psi\) contradictory opposites. If one can deduce such a pair from an assumption \(\theta\), then one can conclude that \(\theta\) is false, or, in other words, one can conclude \(\neg \theta\).