**Euler diagrams for defeasible reasoning**

*Ryo Takemura*

**Abstract.**

We investigate Euler diagrammatic systems for defeasible reasoning by extending the usual systems for Euler and Venn diagrams corresponding to standard classical logic. To achieve this, we use the generalized quantifier â€œmostâ€ to formalize defeasible reasoning, as proposed by Schlechta (1995), where defeasible knowledge is represented as â€œMost A are Bâ€ and axioms for â€œmostâ€ are defined. We introduce an Euler diagrammatic system for defeasible reasoning by introducing circle mA that represents â€œmost Aâ€ for each circle A. We show that our Euler diagrammatic system is a diagrammatic representation of the symbolic system of the generalized quantifier â€œmost.â€ Furthermore, we investigate skeptical and credulous strategies in defeasible reasoning with our Euler diagrams.