The philosophy of mathematics propounded by Brouwer, although heralded in the constructivist views of the mathematicians Leopold Krönecker (1823– 91), Poincaré, and Félix Borel (1871–1956). In reaction against mathematical Platonism, intuitionism holds that the subject-matter of mathematics is the mental constructions of mathematicians. These are thought of as actual processes that have an existence in time, and in Brouwer’s thought connect with the appreciation of time and its division. It was with the development of intuitionistic logic that intuitionism took the stage as a serious constructivist rival to classical mathematics.