The Boltzmann entropy is obtained if one assumes one can treat all the component particles of a thermodynamic system as statistically independent. The probability distribution of the system as a whole then factorises into the product of N separate identical terms, one term for each particle; and when the summation is taken over each possible state in the 6-dimensional phase space of a single particle (rather than the 6 N -dimensional phase space of the system as a whole), the Gibbs entropy