 Arntzenius (1990) points out that the fork asymmetry is highly problematic in the context of classical statistical mechanics within which Reichenbach was working. Let \(\mathcal{S}\) be a state-space containing all possible states of a physical system (see  Figure 7).  The state of a system s at a particular time t will involve the specification of parameters, such as the positions and momenta of all the particles in the system. The system evolves deterministically, so that for every time time t, if the system was in state s at time 0, it will be in state \(U_t(s)\) at time t, where each \(U_t\) is a one-one function from \(\mathcal{S}\) into itself. An event such as C corresponds to a subset of \(\mathcal{S}\): the set of states in which C occurs. We can then define \(U_t(C)\) to be the set of states of the form \(U_t(s)\), where \(s \in C\).