 Section 4 [on negation] departs substantially from classical logic. Here I could not avoid giving the impression that the differences that come to the fore in this section constitute the most important point of conflict between intuitionists and formalists (a claim that is already refuted by the remark made at the beginning [quoted above]); this impression arises because the formalism is unfit to express the more fundamental points of conflict. (Heyting 1930: 44 [Mancosu 1998: 313])