ANDRÉS RIVADULLA THE NEWTONIAN LIMIT OF RELATIVITY THEORY AND THE RATIONALITY OF THEORY CHANGE ABSTRACT. The aim of this paper is to elucidate the question of whether Newtonian mechanics can be derived from relativity theory. Physicists agree that classical mechanics constitutes a limiting case of relativity theory. By contrast, philosophers of science like Kuhn and Feyerabend affirm that classical mechanics cannot be deduced from relativity theory because of the incommensurability between both theories; thus what we obtain when we take the limit c →∞ in relativistic mechanics cannot be Newtonian mechanics sensu stricto. In this paper I focus on the alleged change of reference of the term mass in the transition from one theory to the other. Contradicting Kuhn and Feyerabend, special relativity theory supports the view that the mass of an object is a characteristic property of the object, that it has the same value in whatever frame of reference it is measured, and that it does not depend on whether the object is in motion or at rest. Thus mass preserves the reference through the change of theory, and the existence of a Newtonian limit of relativity theory provides a good example of the rationality of theory change in mathematical physics. – Does mass really depend on velocity, Dad? – No! Well, yes. Actually, no, but don’t tell your teacher. Carl Adler, Am. J. Phys. 55, 1987 1. THE NEWTONIAN LIMIT OF RELATIVITY THEORY AND THE RATIONALITY OF THEORY CHOICE According to Albert Einstein (1917, §§22, 29), the most important aim of a scientific theory is to point to the establishment of a more comprehensive one, in which it survives as a limiting case. For instance, Newton’s gra- vitational theory obtains as a first approximation by specializing general relativity theory’s equations for the case of weak gravitation fields and low velocities. Einstein’s suggestion of the existence of a Newtonian limit is nowadays a universally assumed idea in theoretical physics. Nobel prize winner Lev Landau (1951, §1-1) claims for instance: “The limiting transition from re- lativistic to classical mechanics can be produced formally by the transition to the limit c →∞ in the formulas of relativistic mechanics.” Steven Weinberg (1972, Ch.7) talks about the Newtonian limit of Einstein’s field Synthese 141: 417–429, 2004. © 2004 Kluwer Academic Publishers. Printed in the Netherlands.