et al. [@RecZB] applies.)

On the basis of such experiments it might also be possible to decide whether it is at all possible to perform experiments for measurements of probabilities that cannot be expressed as the expectation of a projector.

Schr[ö]{}dinger’s argument {#schroe}
==========================

In his 1936 paper “Die gegenw[ä]{}rtige Situation in der Quantenmechanik” [@Sch] (where the famous cat paradox appears), Erwin [Schr[ö]{}dinger]{} gives the following argument (pp. 156-157 of the English translation):

*At first thought one might well attempt likewise to refer back the always uncertain statements of Q.M. to an ideal ensemble of states, of which a quite specific one applies in any concrete instance – but one does not know which one. That this won’t work is shown by the one example of angular momentum, as one of many.*

Imagine \[...\] the point $M$ to be situated at various positions relative to $O$ and fitted with various momentum vectors, and all these possibilities combined into an ideal ensemble. Then one can indeed choose these positions and vectors that in every case the product of vector length by length of normal $OF$ \[where $F$ is the point closest to $O$ on the line through $M$ along the momentum 