A Diagrammatic Inference System with Euler Circles ∗ Koji Mineshima, Mitsuhiro Okada, and Ryo Takemura Department of Philosophy, Keio University, Japan. fminesima,mitsu,
[email protected] Abstract Proof-theory has traditionally been developed based on linguistic (symbolic) represen- tations of logical proofs. Recently, however, logical reasoning based on diagrammatic or graphical representations has been investigated by logicians. Euler diagrams were intro- duced in the 18th century by Euler [1768]. But it is quite recent (more precisely, in the 1990s) that logicians started to study them from a formal logical viewpoint. We propose a novel approach to the formalization of Euler diagrammatic reasoning, in which diagrams are defined not in terms of regions as in the standard approach, but in terms of topo- logical relations between diagrammatic objects. We formalize the unification rule, which plays a central role in Euler diagrammatic reasoning, in a style of natural deduction. We prove the soundness and completeness theorems with respect to a formal set-theoretical semantics. We also investigate structure of diagrammatic proofs and prove a normal form theorem. 1 Introduction Euler diagrams were introduced by Euler [3] to illustrate syllogistic reasoning. In Euler dia- grams, logical relations among the terms of a syllogism are simply represented by topological relations among circles. For example, the universal categorical statements of the forms All A are B and No A are B are represented by the inclusion and the exclusion relations between circles, respectively, as seen in Fig. 1. Given two Euler diagrams which represent the premises of a syllogism, the syllogistic inference can be naturally replaced by the task of manipulating the diagrams, in particular of unifying the diagrams and extracting information from them.