The Mechanisation of Inference: Venn’s Logical-Diagram Machine

It is well known that John Venn introduced an ingenious diagrammatic scheme in his 1880 paper “On the Diagrammatic and Mechanical Representation of Propositions and Reasonings.” It is less known that Venn also described there two plans for logical machines, inspired by his diagrams. These machines were said to be analogous to the machine that William S. Jevons constructed a few years earlier. However, Venn had “no high estimate . . . of the interest or importance” of such machines. He argued that such devices perform only a small part of the process required to solve logical problems. Consequently, the help that is offered is very slight. Given that Venn’s machine is founded on his diagrams, one may wonder what Venn’s discussion of logic machines teaches us about his diagrams. The paper argues that Venn failed to notice that his diagrams are vulnerable to the same criticisms that he raised against logic machines.