ArgumentAccording to Hermann von Helmholtz, free mobility of bodies seemed to be an essential condition of geometry. This free mobility can be interpreted either as matter of fact, as a convention, or as a precondition making measurements in geometry possible. Since Henri Poincaré defined conventions as principles guided by experience, the question arises in which sense experiential data can serve as the basis for the constitution of geometry. Helmholtz considered muscular activity to be the basis on which the form of (...) space could be construed. Yet, due to the problem of illusion inherent in the subject’s self-assessment of muscular activity, this solution yielded new difficulties, in that if the manifold is abstracted from rigid bodies which serve as empirical justification of the geometrical notion of space, then illusionary bodies will produce fictive manifolds. The present article is meant to disentangle these difficulties. (shrink)

First publication of a letter of Albert Einstein to Hans Vaihinger, dated May, 1919, concerning Vaihinger's concept of "Fiktionen", the role of Geometry in Poincaré's system, and the importance of "real culture" in the intellectual's style of life. The letter is introduced and annotated.

While the concept of “classical physics” has long framed our understanding of the environment from which modern physics emerged, it has consistently been read back into a period in which the physicists concerned initially considered their work in quite other terms. This essay explores the shifting currency of the rich cultural image of the classical/modern divide by tracing empirically different uses of “classical” within the physics community from the 1890s to 1911. A study of fin‐de‐siècle addresses shows that the earliest (...) general uses of the concept proved controversial. Our present understanding of the term was in large part shaped by its incorporation within the emerging theories of relativity and quantum theory—where the content of “classical” physics was defined by proponents of the new. Studying the diverse ways in which Boltzmann, Larmor, Poincaré, Einstein, Minkowski, and Planck invoked the term “classical” will help clarify the critical relations between physicists’ research programs and their use of worldview arguments in fashioning modern physics. (shrink)

This essay discusses attempts that have been made to explain the striking similarities between two theories propounded in 1905 by Albert Einstein and Henri Poincaré without any mutual reference.