 As an illustration, suppose that a mole of gas is confined to a container of volume \(V_1,\) at pressure \(P_1,\) and temperature \(T_1\). The gas is then allowed to expand isothermally into a larger container of volume \(V_2\). One standard way of explaining the behavior of the gas—its rate of diffusion and its subsequent equilibrium pressure \(P_2\)—appeals to the generalizations of phenomenological thermodynamics—e.g., the ideal gas law, Graham’s law of diffusion, and so on. Salmon appears to regard putative explanations based on at least the first of these generalizations as not explanatory because they do not trace continuous causal processes—he thinks of the individual molecules as causal processes but not the gas as a  whole.[7]  However, it is plainly impossible to trace the causal processes and interactions represented by each of the \(6 \times 10^{23}\) molecules making up the gas and the successive interactions (collisions) it undergoes with every other molecule. Even the usual statistical mechanical treatment, which Salmon presumably would regard as explanatory, does not attempt to do this. Instead, it makes certain general assumptions about the distribution of molecular velocities and the forces involved in molecular collisions and then uses these, in conjunction with the laws of mechanics, to derive and solve a differential equation (the Boltzmann transport equation) describing the overall behavior of the gas. This treatment abstracts radically from the details of the causal processes involving particular individual molecules and instead focuses on identifying higher level variables that aggregate over many individual causal processes and that figure in general patterns that govern the behavior of the gas.