Thermodynamics is the science of how the properties of matter and systems change with temperature. A system may be viewed on the microscopic scale, in which case we study the interactions of the constituent particles or quanta and how these change with temperature. In this approach, we need to construct physical models for these interactions. The opposite approach is to study the system on the macroscopic scale and then the unique status of classical thermodynamics becomes apparent. In this approach, the behaviour of matter and radiation in bulk is studied and in effect we deny that they have any internal structure at all. In other words, the science of classical thermodynamics is solely concerned with relations between macroscopic measurable quantities such as pressure, volume and temperature.

Now this may seem to make classical thermodynamics a rather dull subject but, in fact, it is quite the opposite. In many physical problems, we may not know in detail the correct microscopic physics, and yet the thermodynamic approach can provide answers about the macroscopic behaviour of the system which are independent of the unknown detailed microphysics. Another way of looking at it is to think of classical thermodynamics as providing the boundary conditions which any microscopic model must satisfy. The thermodynamic arguments have absolute validity independent of the model adopted to explain any particular phenomenon.

It is remarkable that these profound statements can be made on the basis of the first and second laws of thermodynamics. Let us state them immediately.