This paper argues that Einstein subscribed to three distinct kinds of interpretations of the quantum theory: subjective, instrumental, and hidden variables interpretations. We explore the context and ihe content of Einstein's thinking over these interpretations, emphasizing Einstein's conception of his role not only as a critic of the new quantum theory but also as a guide pointing the way to better physics.

Besides the well-known advocate of unified field theories, there was “another Einstein,” who was skeptical of the continuum as a foundational element in physics. This paper presents evidence for the existence of this “other Einstein,” and of the debate between the two Einsteins that lasted most of Einstein's life.

Einstein's concept of causality as analyzed in this paper is a thick concept comprised of: (a) regularity; (b) locality; (c) symmetry considerations leading to conservation laws; (d) mutuality of causal interaction. The main theses are: 1. Since (b)–(d) are not elements of Hume's concept of causality, Einstein's concept, the concept embedded in the theory of relativity, is manifestly non–Humean. 2. On a Humean conception, Newtonian mechanics is a paradigmatically causal theory. Einstein, however, regarded this theory as causally deficient, for it fails to comply with both (b) and (d). Special relativity was (partly) motivated by the wish to correct the first of these failures; general relativity the second. 3. Ironically, general relativity, based on the thick concept of causality, opens the way for a conventionalist understanding of that concept. 4. With regard to human freedom, Einstein professed to be a Spinozist. However, he suggested a version of soft determinism, not found in Spinoza.

In this paper I present and argue for a model of conceptual development in science and apply it to the transition from classical to modern physics associated with Einstein. The model claims a continuous and rational transition between incompatible subsequent conceptual systems in mathematical science and explains its mechanism. The model was developed in a study of the transition from preclassical to classical mechanics. I argue for a strong structural analogy between the transition from preclassical to classical mechanics on the one hand and from classical to modern physics on the other. The first transition is briefly sketched here by reference to Galileo and his disciples; in the second transition Planck and Lorentz on the one hand and Einstein on the other play the respective roles.

A detailed and documented reconstruction of the transition from preclassical to classical mechanics on the basis of this model has already been published and is only briefly referred to in the paper. The transition from classical to modern physics is portrayed here much more extensively—though of course merely in broad brush strokes. Einstein–s role in this transition is reconstructed in the light of a conceptualization of his scientific knowledge as an active structure of thought, shaped by his intellectual experience. In this way, the development of his individual thinking is shown to be part of the overall process of conceptual transformation from classical to modern physics. The reconstruction sketched in this paper is to be considered as a proposal to be substantiated, reformed, and improved by future detailed studies.

Carl Gottfried Neumann was born in Königsberg, Prussia, in 1832 and died in Leipzig in 1925. His father was the physicist Franz Neumann (1798–1895), notable for his contributions not only to the study of electricity and magnetism but also to the development of physics education in nineteenth-century Germany. Carl Neumann studied at the University of Königsberg and received his doctorate in 1855 with a work on the application of elliptic integrals to mechanics (Neumann 1856). In 1858 he became Privatdozent, and in 1863 Professor of Mathematics at Halle. Later that same year he moved to Basel, and in 1865 he became Ordinary Professor of Mathematics at Tübingen. Finally in 1868 he was appointed Professor of Mathematics at Leipzig, a post he held until he retired in 1911; of the two mathematics professorships at Leipzig, this was the one formerly held by F. A. Möbius, and it was officially devoted to “the higher mathematics, especially physics” (quoted in Jungnickel and McCormmach 1986, 1:181). So Neumann's academic career, along with his role as one of the founding editors of the Mathematische Annalen beginning in 1869, can be seen as reflecting the enormous advance in mathematical sophistication that German physics underwent in the latter part of the nineteenth century.

If, as is universally acknowledged, the proper goal of the mathematical sciences is the discovery of the least possible number of principles (notably principles that are not further explicable) from which the universal laws of empirically given facts emerge with mathematical necessity, and thus the discovery of principles equivalent to those empirical facts, then it must appear as a duty of indubitable importance to reflect carefully on the principles that have already surfaced with some certainty in one area of the natural sciences and present them in a form that really fulfills the equivalence requirement.