David Albert begins with a brief discussion of an issue raised in the last lecture- whether you can define an entropy for a non-equilibrium system. He and Tim Maudlin agree that while the steps along the reversible route must involve equilbrium states, partitioning can still be used to define the entroy of a non-equilibrium system. Tim returns to the discussion of statistical mechanics began in the last lecture, addressing how we can calculate and explain the velocity distribution of gas particles at equilibrium. Following work by Boltzman and Maxwell, we can model the gas as a system of particles that colide, and calculate the 'critical areas' which particles of a given velocity must be in if they are to colide with other gas particles in a given time. We then have a dynamics for how a velocity distribution will evolve. By assuming the 'Stosszahlansatz'', that the number of particles in the critical areas is proportional to the number of particles in the system, we can show the system will evolve towards a particular distribution.