  To help us in this task, it is useful to note one fact about Newtonian mechanics: the laws are time-reversal invariant (Albert 2000: ch. 1). That is, for every lawful trajectory through the state space, there is another lawful trajectory which results from the first by mapping every instantaneous state in the first trajectory to its image state in which the particles have the same positions but the signs on the components of momentum are reversed, and running the trajectory in reverse order. These image states are those where the particles are in the same positions but moving in exactly the opposite direction. So for every lawful process, that process running backwards is also lawful. (If these trajectories are so lawful, why don’t we see them?—that is the deep question of thermodynamic asymmetry, discussed briefly in supplement  D.)