Using Properties as Variables

Normally, when constructing a mathematical model to be used in the physical sciences, it is desirable that the functions be single-valued in all independent variables and 

The function x = x y, z) in mathematics will always give the same value or values of X for given values of y and z. You will not get one value of x today and a different one tomorrow. You may get two values of x, but you will get both of them today and both tomorrow. When dealing with physical properties, however, it is conceivable that something quite different might happen. We might have, for example, a function relating pressure (P), temperature (T) and volume (V) of a system 

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