  The argument then continues as follows: that a permutation of the particles is counted as giving a different arrangement in classical statistical mechanics implies that, although they are indistinguishable, such particles can be regarded as individuals (indeed, Boltzmann himself made this explicit in the first axiom of his ‘Lectures on Mechanics’, couched in terms of the impenetrability assumption noted above). Since this individuality resides in something over and above the intrinsic properties of the particles in terms of which they can be regarded as indistinguishable, it has been called ‘Transcendental Individuality’ by Post (1963).  This notion can be cashed out in various well-known ways, as indicated in the Introduction above: in terms of some kind of underlying Lockean substance, for example, or in terms of primitive thisness. More generally, one might approach it in modal fashion, through the doctrine of haecceitism: this asserts that two possible worlds may describe some individual in qualitatively the same way (that is, as possessing the same set of properties), yet represent that individual differently by ascribing a different haecceity or thisness in each world, or more generally, by ascribing some non-qualitative aspect to the individual (Lewis 1986; Huggett 1999a).