  How may they be justified and explained? In the discussions concerning the problem of irreversibility that ensued after Boltzmann's work, attention was focussed on a fundamental assumption he made: the hypothesis with regard to collision numbers. This time-asymmetrical assumption posited that the motions of the molecules in a gas were statistically uncorrelated prior to the molecules colliding. In deriving any of the other kinetic equations a similar such posit must be made. Some general methods for deriving such equations are the master equation approach and an approach that relies upon coarse-graining the phase space of points representing the micro-states of the system into finite cells and assuming fixed transition probabilities from cell to cell (Markov assumption). But such an assumption was not derived from the underlying dynamics of the system, and, for all they knew so far, might have been inconsistent with that dynamics.