The question under investigation is: Can the assumption that some observables of some physical systems (including measuring instruments)have in all circumstances definite values be made compatible with the linear laws of evolution of quantum mechanics by means of the hypothesis that not all Hermitian operators of a macrosystem correspond to observable quantities? To that effect the meaning of the assumption under study is clarified (Sect.1) and the conditions under which a retrodictive calculation should be considered as reliable are examined (Sect.2). The results can then be applied to the problem at hand (Sect.3). With proper specifications, the conclusion is negative. This is interpreted as being simply a further corroboration—in quantum measurement theory—of a well-known fact, which is that any fundamental reality that is assumed to lie beyond the concepts of a merepractical realism and explain them must be considered as being nonseparable.