Nelson's Logical Diagrams

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 Blanché . 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.