 When the localization frequency is related to the mass of the constituents, then, in completely generality (i.e., even when one is dealing with a body which is not almost rigid, such as a gas or a cloud), the mechanism leading to the suppression of the superpositions of macroscopically different states is fundamentally governed by the the integral of the squared differences of the mass densities associated to the two superposed states. Actually, in the original paper the mass density at a point was identified with its average over the characteristic volume of the theory, i.e., \(10^{-15}\) cm\(^3\) around that point. It is however easy to convince oneself that there is no need to do so and that the mass density at any point, directly identified by the statevector (see below), is the appropriate quantity on which to base an appropriate ontology. Accordingly, we take the following attitude: what the theory is about, what is real ‘out there’ at a given space point \(\boldsymbol{x}\), is just a field, i.e., a variable \(m(\mathbf{x},t)\) given by the expectation value of the mass density operator \(M(\boldsymbol{x})\) at \(\boldsymbol{x}\) obtained by multiplying the mass of any kind of particle times the number density operator for the considered type of particle and summing over all possible types of particles which can be present: