  We discuss the generic definition of the $\tau$ function for the arbitrary
Hamiltonian system. The different approaches concerning the deformations of the
curves and surfaces are compared. It is shown that the Baker-Akhiezer function
for the secondary integrable system of the Toda lattice type can be identified
with the coherent wave function of the initial dynamical system. The $\tau$
function appears to be related to the filling of the interior of the classical
trajectory by coherent states. Transition from dispersionless to dispersionful
Toda lattice corresponds to the quantization of the initial dynamical system.
