  Third, and most importanty, the main weakness of the present result is its assumption that the trajectory actually visits all points on the energy hypersurface.  This is what the Ehrenfests called the ergodic  hypothesis.[8]  Boltzmann returned to this issue on the final page of the paper (WA I, 96). He notes there that exceptions to his theorem might occur, if the microscopic variables would not, in the course of time, take on all values compatible with the conservation of energy. For example this would be the case when the trajectory is periodic. However, Boltzmann observed, such cases would be immediately destroyed by the slightest disturbance from outside, e.g., by the interaction of a single external atom. He argued that these exceptions would thus only provide cases of unstable equilibrium.