7. The War Years

Cut off from his international contacts, in particular Göttingen, Brouwer returned to his foundational research. He lectured on The theory of point sets, a course on constructive set theory. In the 1916/17 course he introduced choice sequences and the continuity principle. Now that he saw how to exploit the apparent weakness of his intuitionistic mathematics, he decided to build his new intuitionism in a systematic way. There were some signs during the war years that something was brewing, but the first papers appeared after the war. Part of his efforts were directed at a project called Significs, a study of language and meaning following Lady Welby and Frederik van Eeden, the author and first psychiatrist in Holland. A considerable effort was made to create a group of people with a common interest in the subject, including an International Academy, etc. In the later years of the war Brouwer proposed, together with colleagues, to found a section of the Dutch Airforce (which was hardly existing at the time) for the study and application of air reconnaissance (photogrammetry). Finally, from now on Brouwer was spending more time on university/faculty/Academy matters.