 Physics treats systems by assigning them states. The thermodynamic state of a gas specifies its pressure, volume and temperature. The state of a system of classical particles is represented as a point in a phase space coordinatized by their positions and momenta. One expects that if a physical system is composed of physical subsystems, then both the composite system and its subsystems will be assigned states by the relevant physical theory. One further expects that the state of the whole will not be independent of those of its parts, and specifically that if a system is composed of two subsystems, A and B, then it will satisfy a principle formulated by Einstein (1935). Howard (1985, p.180) gives the following translation of this principle, which I’ll call the