  A new approach to obtain the closure formulas for the von K\'arm\'an--Howarth
and Corrsin equations is presented, which is based on the Lagrangian
representation of the fluid motion, and on the Liouville theorem associated to
the kinematics of a pair of fluid particles. This kinematics, characterized by
the finite--scale separation vector, is assumed to be statistically independent
from the velocity field. Such assumption is justified by the hypothesis of
fully developed turbulence and by the property that this vector varies much
more rapidly than the velocity field. This formulation leads to the closure
formulas of von K\'arm\'an--Howarth and Corrsin equations in terms of
longitudinal velocity and temperature correlations following a demonstration
completely different with respect to the previous works. Some of the properties
and the limitations of the closed equations are discussed. In particular, the
times of evolution of the developed kinetic energy and temperature spectra are
shown to be finite quantities which depend on the initial conditions.
