I consider the conceptual arguments leading to the validity of the conventional physical approach to thermodynamics, as we know it. In spite of the familiarity we have with this approach, its internal logical structure deserves to be extracted. No new features are found, but some universal properties emerge. These are then applied to the unconventional system of a black hole, and its thermodynamic description is considered in view of the universal framework presented. It is found that the black hole is highly exceptional, and its thermodynamics contains some unexpected and questionable results. We also discuss the role of entropy as residual indeterminess when a physical system is described by a reduced theory.