  The review paper presents generalization of d'Alembert's variational
principle: the dynamics of a quantum system for an external observer is defined
by the exact equilibrium of all acting in the system forces, including the
random quantum force $\hbar j$, $\forall\hbar$. Spatial attention is dedicated
to the systems with (hidden) symmetries. It is shown how the symmetry reduces
the number of quantum degrees of freedom down to the independent ones. The
sin-Gordon model is considered as an example of a such field theory with
symmetry. It is shown why the particles $S$-matrix is trivial in that model.
