  In quantum statistical mechanics, however, we have two ‘standard’ forms: one for which there are three possible arrangements in the above situation (both particles in one box, both particles in the other, and one in each box), giving ‘Bose-Einstein’ statistics; and one for which there is only one arrangement (one particle in each box), giving ‘Fermi-Dirac’ statistics (which underpins the Pauli Exclusion Principle and all that entails). Setting aside the differences between these two kinds of quantum statistics, the important point for the present discussion is that in the quantum case, a permutation of the particles is not regarded as giving rise to a new arrangement. This result lies at the very heart of quantum physics; putting things slightly more formally, it is expressed by the so-called ‘Indistinguishability Postulate’: