 Intuitively, \( \neg (\phi \wedge \psi) \) intuitionistically asserts that an effective method has been given, which would convert any proof of \((\phi \wedge \psi)\) into a proof of a contradiction. But this does not imply that either a proof of \(\neg \phi\) (i.e., an algorithm which would convert any proof of \(\phi\) into a proof of a contradiction) or a proof of \(\neg \psi\) has been given, which is what \((\neg \phi \vee \neg \psi)\) intuitionistically asserts.