 There is another aspect of this example that is worthy of comment. Suppose that a particular sample of gas expands in a way that meets the conditions described above and that it is somehow possible to provide an account that traces each of the individual molecular trajectories of its component molecules. Such an account would nonetheless leave out information that seems explanatorily relevant. This information has to do with the fact that there are a large number of alternative trajectories besides the actual trajectories followed by the component molecules on this particular occasion that would lead to the same final pressure \(P_2\) which is what we want to explain. Modal information of this sort—both about what would happen if the molecules followed different trajectories consistent with the initial conditions \(P_1\), \(V_1\) and \(T_1\) and what would happen if they instead followed initial conditions consistent trajectories consistent with different macroscopic initial conditions is better captured by a treatment in terms of upper level thermodynamic variables.