 Suppose that a microsystem \(S\), immediately before the measurement of one of its observables, say \(B\), is in the eigenstate \(\ket{b_j}\) of the corresponding operator. The apparatus (a macrosystem) used to gain information about \(B\) is initially assumed to be in a precise macroscopic state, its ready state, corresponding to a definite macro property—e.g., its pointer points at 0 on a scale. Since the apparatus \(A\) is made of elementary particles, atoms and so on, it should be possible to describe it within quantum mechanics, which will associate a well defined state vector \(\ket{A_0}\) to it (at least in principle). One then assumes that there is an appropriate system-apparatus interaction lasting for a finite time, such that when the initial state of the apparatus is triggered by the state \(\ket{b_j}\), it ends up in a final configuration \(\ket{A_j}\), which is macroscopically distinguishable from the initial one and from the other configurations \(\ket{A_k}\) in which it would end up if triggered by a different eigenstate \(\ket{b_k}\). Moreover, for simplicity one assumes that the system is left by the measurement in its initial state. In brief, one assumes that one can dispose things in such a way that the system-apparatus interaction can be described as: