When dealing with n-particle systems that are identical, is the superposition of them just a mathematical construct, or is it similar to how the state of a single particle can be in multiple eigenstates until its measured. For instance, if I have two fermions: [itex]\Psi = \Psi_a(x_1)\Psi_b(x_2) - \Psi_b(x_1)\Psi_a(x_2)[/itex] then are we describing it in this way only because we don't know which state it's in? Not necessarily because the state is in both configurations prior to measure? And moreover, what does subtraction mean here? How can you subtract two states?