  The result was, however, that Boltzmann rethought the basis of his approach and in 1877b produced a conceptually very different analysis, which might be called the combinatorial argument, of equilibrium and evolutions towards equilibrium, and the role of probability theory. The distribution function, which formerly represented the probability distribution, was now conceived of as a stochastic variable (nowadays called a macrostate) subject to a probability distribution. That probability distribution was now determined by the size of the volume in phase space corresponding to all the microstates giving rise to the same macrostate, (essentially given by calculating all permutations of the particles in a given macrostate). Equilibrium was now conceived of as the most probable macrostate instead of a stationary macrostate. The evolution towards equilibrium could then be reformulated as an evolution from less probable to more probable states.