  We summarize the papers published by Einstein in the Annalen der Physik in
the years 1902-04 on the derivation of the properties of thermal equilibrium on
the basis of the mechanical equations of motion and of the calculus of
probabilities. We point out the line of thought that led Einstein to an
especially economical foundation of the discipline, and to focus on
fluctuations of the energy as a possible tool for establishing the validity of
this foundation. We also sketch a comparison of Einstein's approach with that
of Gibbs, suggesting that although they obtained similar results, they had
different motivations and interpreted them in very different ways.
