 Mannoury (1867–1956) was a philosopher, mathematician, psychoanalyst, accountant, and political activist (Kirkels, 2019). Largely an autodidact, it was he who introduced in the Netherlands both topology (in a series of papers of 1897–1900) and Peano’s symbolic logic (in a lecture in 1903). In 1903 he was appointed privaatdocent (unpaid lecturer) in the logical foundations of mathematics at the University of Amsterdam, where Brouwer was among his first and most enthusiastic students. His lectures have been published, in shortened and revised form, as Mannoury, 1909. But it was Mannoury’s earlier, topological papers that influenced the young Brouwer decisively: the “undertone” of that work, Brouwer said in a speech in 1946, had changed his view of mathematics from a collection of “truths fascinating by their immovability, but horrifying by their lifelessness, like stones from barren mountains of disconsolate infinity” to a concern with what is “built ... out of the structural elements of our thinking” (Brouwer, 1946B, pp. 192–193). Brouwer was also very interested in Mannoury’s reflections on the uses and misuses of language in mathematics and philosophy. In 1917 Mannoury became professor extraordinarius, in 1918 ordinarius at the University of Amsterdam, succeeding Korteweg at the chair of analytic and descriptive geometry, mechanics and philosophy of mathematics, until 1937. A large part of his career was devoted to furthering the development of significs, an analysis of communicative acts starting from the ideas laid down by Victoria, Lady Welby in her paper “Sense, Meaning and Interpretation” (Welby, 1896). Brouwer was also involved in that project.