Johnson around the turn of the twentieth century. Historically speaking, it has long been assumed that this type of octagon was studied for the first time by John Neville Keynes and William E.
Throughout the twentieth century, this type of diagram has been studied quite extensively in philosophical logic, and in recent years even in computer science. Based on a detailed analysis of this octagon, in combination with De Morgan’s unpublished notes as well as his published materials, we argue that this diagram belongs to the type of so-called KJ octagons. We will focus on one of De Morgan’s unpublished diagrams, which occurs several times throughout his manuscripts.
In this paper, we present some archival findings and discuss their contemporary relevance. Although there are hardly any diagrams or figures in De Morgan’s published writings, in his unpublished manuscripts, one can find various attempts to draft certain figures of opposition, which are evidently meant to fit the relations between the propositions of De Morgan’s extended syllogistics. The British logician Augustus De Morgan (1806–1871) sought to unify the traditional syllogistics with the new algebraic logic that he and George Boole (1815–1864) were developing.
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