Volume 20, Issue 4, December 1997
Peirce's Existential Graphs
A Practical Alternative to Truth Tables for Critical Thinkers
Teachers of critical thinking courses are justified in teaching some amount of propositional logic, especially logical equivalence and formal proofs for validity, but the otherwise informal nature of most critical thinking courses makes it difficult to decide how much propositional logic should enter a course. Most instructors use truth tables to teach the above two topics but they are too off-putting to be useful to most critical thinking students (as are derivations, the common alternative to truth tables). This paper presents an accessible alternative to truth tables, namely, Peirce’s Existential Graphs. Detailing only what one needs to know about Existential Graphs in order to test propositions for logical equivalence and deductive argument forms for validity, this paper describes how they work, their terminology, their inference rules, and several examples which may aid in demonstrating their use to students.