 A further likely influence is Brouwer’s unpublished elucidation of the virtual ordering axioms (see  section 3.1.3  above). Dirk van Dalen (personal communication) suspects that, although Heyting was probably not present at this lecture course, he heard Brouwer make a similar comment on another occasion (for example during the years 1922–1925, when Heyting was working on his dissertation under Brouwer, for that work also considers intuitionistic orderings).