  In this paper we propose a new way of organizing many-body perturbation
theory in the Path-integral formulation where a set of quasi-particle
wave-functionals $\psi$'s are introduced and are identified with
quasi-particles in Landau Fermi liquid theory. We show how Fermi liquid theory
can be obtained through $\psi$'s in this new framework of perturbation theory,
where the only assumption is adiabaticity between non-interacting and
interacting states and the quasi-particle renormalization factor z does not
appear explicitly. Consequences of our new formulation are discussed.
