The discussion begun in the preceding chapters raises the further question of how quantum mechanics is related to classical mechanics. We have insisted that quantum mechanical variables are differently defined from those of classical physics, not only because of the noncommutation of operators, but also because quantum mechanics includes in an organic way the functions of both a deterministic and statistical classical theory. Quantum and classical physics, and the variables appropriate to each are analogous to one another. We shall now examine the analogues which exist between these two branches of physics, and explain in what way classical mechanics is a limiting case of quantum mechanics.