‘Like thermodynamics before Boltzmann.’ On the emergence of Einstein's distinction between constructive and principle theories

In a 1919 article for the Times of London, Einstein declared the relativity theory to be a ‘principle theory’, like thermodynamics, rather than a ‘constructive theory’, like the kinetic theory of gases. The present paper attempts to trace back the prehistory of this famous distinction. It provides a systematic overview of Einstein's repeated use of the relativity theory/thermodynamics analysis after 1905 and inserts it into its historical settings, the early reception of relativity against the background of fin de siècle electron theories. Einstein initially used the relativity theory/thermodynamics comparison to address a specific objection. In his 1905 relativity paper he had determined the velocity-dependence of the electron's mass by adapting Newton's particle dynamics to the relativity principle. However, according to many, this result was not admissible without making some assumption about the structure of the electron. Einstein replied that the relativity theory is similar to thermodynamics. Unlike the usual physical theories, it does not directly try to construct models of specific physical systems; it provides empirically motivated and mathematically formulated criteria for the acceptability of such theories. New theories can be obtained by modifying existing theories valid in limiting case so that they comply with such criteria. Einstein progressively transformed this line of the defense into a positive heuristics. Instead of directly searching for new theories, it is often more effective to search for conditions that constraint the number of possible theories. The constructive/principle theories opposition should be considered not only as abstract classification of theories, but also as Einstein's attempt to formulate a sort of ‘logic of discovery’. The paper argues that most of Einstein's scientific successes were obtained by following the principle strategy. Most of his failures happened when he was forced to fall back to the constructive strategy.