Response to Saunders, ‘On the Emergence of Individuals’

Simon Saunders’ work on identity in physics has been groundbreaking. In his most recent article, he goes against his well-known view that particles are weak discernibles to argue that, in fact, they are often individuals. But this individuality emerges from the idiosyncratic structure of the physical states, and so cannot be assumed at the outset; hence the continued need for a formalism that imposes permutation-invariance and which posits weak discernibles. In this response article, I hope mainly to develop Saunders’ conclusions in the spirit of his project, although I object to Saunders’ continued contention, inspired by the Hilbert-Bernays-Quine “reduction” of identity, that objects must be at least weakly discernible. First I will argue that permutation-invariance is best construed along the lines of other invariance principles familiar in gauge theories. In the case of permutation-invariance, the “gauge quantities” in question are the particle labels and their associated formal apparatus. Second, I will present a couple of results regarding the emergence of individuality for quantum particles, particularly fermions. I argue that these results show that: (i) individuality emerges for fermions in every joint state, but for bosons only in some joint states; and that (ii) it is paradoxically under-determined, given a fermionic joint state, which individuals the joint state decomposes into—a situation that owes nothing to entanglement. In this sense, the news regarding fermionic individuality is both better and worse than Saunders suggests. ∗[email protected]