  There is a major confusion among modern commentators about the role and status of the ergodic hypothesis in Boltzmann's thinking. Indeed, the question has often been raised how Boltzmann could ever have believed that a trajectory traverses all points on the energy hypersurface, since, as the Ehrenfests conjectured in 1911, and was shown almost immediately in 1913 by Plancherel and Rozenthal, this is mathematically impossible when the energy hypersurface has a dimension larger than 1.