 for every pair of vectors \(\Psi_{p}, \Psi_{e}\) in \(H_{p}\), \(H_{e}\) respectively. This definition naturally generalizes to systems composed of n distinguishable particles. But alternative definitions seem preferable for a collection of indistinguishable particles—of electrons or of photons for example (see Ghirardi et al. (2002), Ladyman et al. (2013)).