  First, the difference between the approach relying on the ergodic hypothesis and that relying on the SZA is rather striking. Instead of concentrating on a specific gas model, Boltzmann here assumes a much more general model with an arbitrary interaction potential \(V(\vec{q}_1,\ldots,\vec{q}_N)\).  Moreover, the probability density \(\rho\) is defined over phase space, instead of the space of molecular velocities. This is the first occasion where probability considerations are applied to the state of the mechanical system as whole, instead of its individual particles. If the transition between kinetic gas theory and statistical mechanics may be identified with this caesura, (as argued by the Ehrenfests and by Klein) it would seem that the transition has already been made right here in 1868, rather than only in 1877. 