Peircean Graphs for Prepositional Logic
The contributions of C.S. Peirce to the early history of prepositional and predicate logic are well known. Much less well known, however, is Peirce’s subsequent work (for more than ten years) on diagrammatic versions of prepositional and predicate logic. This work was considered by Peirce himself to be his most important contribution to logic. From his experience with chemistry and other parts of science, Peirce had become convinced that logic needed a more visually perspicuous notation, a notation that displayed the compound structure of propositions the way chemical diagrams displayed the compound structure of molecules. Peirce’s graphs for prepositional sentences are built up from sentence letters by the operations of enclosing a graph within a closed figure (interpreted as its negation) and juxtaposing two or more graphs on separate parts of the page (interpreted as their conjunction). For example, a graph like would be interpreted as “If A and B, then C” or “It is false that A and B and not-CV’ A graph such as would be read as “If A then B, but not C.” This chapter gives a modern analysis of Peirce’s diagrammatic version of the propositional calculus from the ground up. In particular, it is an investigation of just what is involved in formulating precisely the syntax, semantics, and proof theory of Peirce’s graphical approach to propositional logic. The project is interesting for several reasons. First, given Peirce’s importance in the history of .logic and his own opinion of the value of his work on graphs, it seems of historical interest to see to what his suggestions amounted. Second, Peirce’s work has gained a following in the computer science community, due especially to the work of Sowa [1984], whose system of conceptual graphs is modeled after Peirce’s work on diagrammatic approaches to prepositional and predicate logic. Third, in reconstructing Peirce’s graphical system we will confront a number of features characteristic of “visual” or “diagrammatic” inference, that is, inference that employs various forms of graphical representations in addition to, or in place of, sentences.