 Later, Brouwer pointed out the following consequence of the validity of \(\neg \neg \neg A \leftrightarrow \neg A\): the proof method of reductio ad absurdum can be used to establish negative propositions \(\neg A\) (Brouwer 1929A: 163 [Mancosu 1998: 52]). For if the assumption of \(\neg \neg A\) leads to a contradiction, that is, to \(\neg \neg \neg A\), the equivalence allows one to simplify that to \(\neg A\). On the other hand, reductio ad absurdum in general cannot be used to establish positive propositions \(A\); the derivation of a contradiction from the assumption \(\neg A\) only leads to \(\neg \neg A\), which intuitionistically is weaker than \(A.\)