ABSTRACT
This chapter describes the Einstein's contributions to early quantum theory are the result of a decade of insights provided by wielding Boltzmann's principle as a probe into the obscure realm of the quantum. The old quantum theory might be then deemed a post-classical theory, but properly such a designation can be applied only retrospectively, i.e., after the rise of quantum mechanics. With Boltzmann's principle as guide, an expanded thermodynamics could provide a reliable epistemic footing upon which to build amidst all other theoretical uncertainty. Einstein considered a system composed of two connected parts, one with variable energy E in thermal equilibrium at absolute temperature T with a second much larger system. Einstein's use of Boltzmann's principle, with the requisite physical conception of probability, freed statistical mechanics of its mechanical presuppositions, even in the supposedly purely statistical form of Boltzmann 1877.
