  We propose a novel method of evaluating the effective action, wherein the
physical one- and two-point functions are obtained in the limit of
non-vanishing external sources. We illustrate the self-consistency of this
method by recovering the usual 2PI effective action due to Cornwall, Jackiw and
Tomboulis, differing only by the fact that the saddle-point evaluation of the
path integral is performed along the extremal quantum, rather than classical,
path. As such, this approach is of particular relevance to situations where the
dominant quantum and classical paths are non-perturbatively far away from
one-another. A pertinent example is the decay of false vacua in
radiatively-generated potentials, as may occur for the electroweak vacuum of
the Standard Model. In addition, we describe how the external sources may
instead be chosen so as to yield the two-particle-point-irreducible (2PPI)
effective action of Coppens and Verschelde. Finally, in the spirit of the
symmetry-improved effective action of Pilaftsis and Teresi, we give an example
of how the external sources can be used to preserve global symmetries in
truncations of the 2PI effective action. Specifically, in the context of an
O(2) model with spontaneous symmetry breaking, we show that this approach
allows the Hartree-Fock approximation to be re-organized, such that the
Goldstone boson remains massless algebraically in the symmetry-broken phase and
we obtain the correct second-order thermal phase transition.
