 Reichenbach saw his fork asymmetry as a macro-statistical analog of the second law of thermodynamics. Suppose we have a system such as a gas that is made up of a large number of particles. Each particle can be in one of many possible states \(s_1,\dots,s_n\). Let \(p_i\) be proportion of the particles that are in state \(s_i\). Then one expression for the entropy of the system is \(S = {-}\sum_i p_i log(p_i)\). This sum will reach a maximum when the particles are evenly distributed among the n states (or distributed as evenly as possible given constraints on the system). The second law of thermodynamics states that the entropy of a closed system can increase but never decrease; hence it will evolve toward a state of maximum entropy. Reichenbach claims that if we find a closed system in a state of low entropy, we can infer that it was recently prepared in that state, before being closed off from the rest of the environment.