  As is now well known, the generalization of the square of opposition to a
hexagon was discovered independently by three philosophers in the early 1950s:
Paul Jacoby, Augustin Sesmat, and Robert Blanch\'{e} . Much less well known is
the earlier use of similar diagrams by the German philosopher Leonard Nelson
(1882-1927). This paper explores the significance of Nelson's work for the
history of the JSB hexagon and for logical geometry more widely.
