Here, the concept of indistinguishability is defined for identical particles by the symmetry of the state, therefore applying to both the classical and the quantum framework. The author describes symmetric statistical operators and classifies these by means of extreme points. He derives de Finettis theorem for the description of infinitely extendible interchangeable random variables, and presents generalisations covering the Poisson limit and the central limit. Finally, a characterisation and interpretation of the integral representations of classical photon states in quantum optics are...