Einstein ’ s 1935 Derivation of E � mc 2

Einstein’s 1935 derivation of mass—energy equivalence is philosophically impor­ tant because it contains both a criticism of purported demonstrations that proceed by analogy and strong motivations for the definitions of the ‘new’ dynamical quantities (viz relativistic momentum, relativistic kinetic energy and relativistic energy). In this paper, I argue that Einstein’s criticism and insights are still relevant today by showing how his derivation goes beyond Friedman’s demonstration of this result in his Foundations of Spacetime 1heories. Along the way, I isolate three distinct physical claims associated with Einstein’s famous equation that are some­ times not clearly distinguished in philosophical discussions of spacetime theory.