Geometry, Logic, and Philosophy: The Case of the Parallels Postulate

Speaker: Patricia Blanchette

Date: Thu, Mar 18, 2021 to Fri, Mar 19, 2021

Location: Zoom, PIMS, University of Calgary

Conference: The Calgary Mathematics & Philosophy Lectures

Subject: Mathematics, Logic and Foundations

Class: Scientific


One of the most important techniques provided by modern logic is the use of models to show the consistency of theories. The technique burst onto the scene in the late 19th century, and had its most important early instance in demonstrating the consistency of non-Euclidean geometries. This talk investigates the development of that technique as it transitions from a geometric tool to an all-purpose tool of logic. I’ll argue that the standard narrative, according to which our modern technique provides answers to centuries-old questions, is mistaken. Once we understand how modern models work, I’ll argue, we see important differences between the kinds of consistencyclaims that would have made sense e.g. to Kant and the kinds of consistency-claims that we can demonstrate today. We’ll also see some philosophically-interesting shifts, over this time period, in the kinds of things that we take proofs to demonstrate.


Patricia Blanchette is Professor of Philosophy and Glynn Family Honors Collegiate Chair in the Department of Philosophy at the University of Notre Dame. Prior to coming to Notre Dame, Blanchette taught in the Department of Philosophy at Yale University. Blanchette works in the history and philosophy of logic, philosophy of mathematics, history of analytic philosophy, and philosophy of language. She is an editor of the Bulletin of Symbolic Logic, and serves on the editorial boards of the Notre Dame Journal of Formal Logic and of Philosophia Mathematica. She is the author of Frege’s Conception of Logic (Oxford University Press 2012).