  We discuss some new aspects of the theory of the Jimbo-Miwa-Ueno tau function
which have come to light within the recent developments in the global
asymptotic analysis of the tau functions related to the Painlev\'e equations.
Specifically, we show that up to the total differentials the logarithmic
derivatives of the Painlev\'e tau functions coincide with the corresponding
classical action differential. This fact simplifies considerably the evaluation
of the constant factors in the asymptotics of tau-functions, which has been a
long-standing problem of the asymptotic theory of Painlev\'e equations.
Furthermore, we believe that this observation is yet another manifestation of
L. D. Faddeev's emphasis of the key role which the Hamiltonian aspects play in
the theory of integrable system.
  This article will appear in the WSPC memorial volume dedicated to Ludwig
Faddeev.
