We analyze the evolution of EHRENFEST's thought since he proved the necessity of quanta in 1911 until the formulation of his adiabatic hypothesis in 1914. We argue that his research contributed significantly to the solution of critical problems in quantum physics and led to a rigorous definition of the range of validity of BOLTZMANN's principle.

This paper is a preliminary exploration of the connections between the statistical style of reasoning and the research practices of statistical mechanics in the early period of the long quantum revolution. It suggests that before 1925 the instantiations of the statistical style in physics went through two phases. The first phase consisted of the formulation of the Maxwell‐Boltzmann statistics on the basis of the population‐gas analogy. The second phase was characterized by the generalization of the Maxwell‐Boltzmann statistics through analogies between (...) ideal gas molecules and other microphysical entities, analogies that shaped and were shaped by the rise of quantum theory. Einstein's invention of the Bose‐Einstein statistics started a third phase and created the conditions of possibility for a new classification of microphysical entities according to their different statistics. (shrink)