 The systems introduced by Frege and Russell were systems of classical logic, but soon after systems of non-classical logics were considered by other logicians. The first influential attempts to introduce logics different from classical logic remained within the Frege-Russell tradition of presenting a logical deduction system without any formal semantics. These attempts lead to the first modal systems of C.I. Lewis (1918, 1932) and to the axiomatization of intuitionistic logic by Heyting (1930).