Thermodynamics as a Mathematical Model

It follows from this that when the gas is confined to the smaller volume it has one value of the entropy, when the wall is pierced it has another value, and that it is the act of piercing the wall and not the subsequent expansion that increases the entropy. In the same way when two bodies at different 

Because we have not yet introduced the entropy this quotation may seem out of place, but in fact, as also pointed out by Pippard, the concept applies equally well to any thermodynamic variable. For example, one could say that it is the piercing of the wall and not the subsequent expansion that changes the volume of the gas. By this we mean that the thermodynamic variable volume is fixed by the physical constraints on the system, and once these are fixed, the (model) volume is fixed, whether the gas has achieved this volume or not. In other words, the properties considered by thermodynamics belong to the model, not to the system being modeled. 

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