  In fact, PII(2) and PII(3) are clearly violated in classical physics, where distinct particles of the same kind are typically regarded as indistinguishable in the sense of possessing all intrinsic properties in common and such properties are regarded as non-relational in general and non-spatio-temporal in particular. (Of course, Leibniz himself would not have been perturbed by this result, since he took the Principle of Identity of Indiscernibles to ultimately apply only to ‘monads’, which were the fundamental entities of his ontology. Physical objects such as particles were regarded by him as merely ‘well founded phenomena’.) However, PII(1) is not violated classically, since classical statistical mechanics typically assumes that such particles are impenetrable, in precisely the sense that their spatio-temporal trajectories cannot overlap. Hence they can be individuated via their spatio-temporal properties, as indicated above.