I was reading Thermodynamics by Herbert Callen. In the first chapter he makes the following statement. 'The basic problem of thermodynamics is the determination of the equilibrium state that eventually results after the removal of internal constraints in closed composite system.' Then he postulates 'There exists a function (called the entropy) of the extensive parameters of any composite system, defined for all equilibrium states and having the following property. The values assumed by the extensive parameters in the absence of an internal constraint are those that maximise the entropy over the manifold of constrained equilibrium states.' In the first statement, what does he mean when he says 'the basic problem'. Does he mean that every problem in (equilibrium) thermodynamics can be reduced to the above problem? If not, why is this problem so important as to postulate entropy on its basis?