The project provides an analysis of the meaning(s) of ‘fallacy’ at the interface of psychology/cognitive science, logic and mathematics, law and argumentation theory, rhetoric and communication studies. We combine conceptual analysis with a spatial model of representation, and account for semantic information by starting from truth-conditions, then transfer this information to a geometric framework using conceptual spaces theory. Project results constitute boundary work across the academic fields in which ‘fallacy’ is presently used with distinct, yet historically and systematically related meanings; secondary use occurs in educational contexts. An application-instance of philosophical methods of analysis and representation to semantic information, the project extends the applicant’s prior work to a new research area; the project structure may readily be applied to other crucial terms and concepts.