Are quantum particles objects?

It is widely believed that particles in quantum mechanics are metaphysically strange; they are not individuals (the view of Cassirer 1956), in some sense of the term, and perhaps they are not even objects at all, a suspicion raised by Quine(1976a, 1990). In parallel it is thought that this dierence, and es- pecially the status of quantum particles as indistinguishable, accounts for the dierence between classical and quantum statistics - a view with long historical credentials.1 'Indistinguishable'here mean permutable; that states of aairs diering only in permutations of particles are the same - which, satisfyingly, are described by quantum entanglements, so clearly in a way that is conceptually new. And, indeed, distinguishable particles in quantum mechanics, for which permutations yield distinct states, do obey classical statistics, so there is something to this connection. But it cannot be the whole story if, as I will argue, at least in one notable tradition, classical particle descriptions may also be permutable (so classical particles may also be counted as indistinguishable); and if, in that same tradi- tion, albeit with certain exceptions, quantum particles are bona …de objects. 1 I will follow Quine in a number of respects, …rst, with respect to the formal, metaphysically thin notion of objecthood encapsulated in the use of singular terms, identity, and quanti…cation theory; second (Quine 1970), in the appli- cation of this apparatus in a …rst-order language L, and preferably one with only a …nite non-logical alphabet; and third (Quine 1960), in the use of a weak version of the Principle of Identity of Indiscernibles (PII). Applying the latter requires a listing of the allowable predicates (the non-logical vocabulary of L); for our present purposes this should be dictated on theoretical and experimental grounds, grounds internal to the physics - for example, that only predications of measurable properties and relations should be allowed. Our minimal, logical question is then: whether indistinguishable quantum particles are L-discernible by their measurable properties and relations. But as a criterion for membership in L, measurability may be somewhat too restrictive; it threatens to settle our question, negatively, solely on the basis that quantum particles are unobservable. Better is a condition that is both precise and more general, namely that only predicates invariant under the symmetries of the theory qualify. This condition implicitly or explicitly underlines a good many recent debates in the philosophy of physics over symmetry principles,