ABSTRACT
This chapter argues that the classical theory has conceptual difficulties that are resolved by quantum mechanics. Since the classical theory from a fundamental point of view is a special case of the quantum theory at large quantum numbers, it is very natural that there remains traces of the quantum treatment in the classical case. In classical mechanics, the Hamiltonian of a system is equal to the total energy, that is, the sum of the kinetic and potential energy of all particles in the system. Other thermodynamical quantities can also be expanded in a power series in density. The probability to observe a certain kinetic state of the system is completely independent of the interactions; it is the same for strongly interacting molecules in a liquid as it is for a thin gas of the same molecules at the same temperature.
