ABSTRACT
This chapter focuses on the notions of classical instruments and of classical measurements. It reviews the theories, based on the concept of macroscopic properties, that is, on the essential complexity of the measuring apparatus. One of the most distinctive features of quantum mechanics is the fact that it often assigns precise structures—or discrete "choices" among possible different structures—to the physical systems it describes. Hisenberg's uncertainty relations bear on the dispersions of ensembles as a whole, whereas the limitations that serve to define "classical measurements" essentially concern our possibilities of actually performing measurements in different elements of the ensemble. Thus from the uncertainty relations alone one can certainly not derive the conclusion that nonclassical measurements are impossible. The chapter considers the restricted set of observables. It is convenient to use the word "macrostate" to designate the whole set of microstates on which all the classical observables forming a definite complete set have definite values.
