Abstract  A phenomenological expression for the vacuum amplitude of gravitons is found in the framework of Schwinger's source theory. The expression provides the basis for a formulation of the correspondence principle and solution of the energy-momentum problem in gravitation theory. Theorems on the conservation of higher moments are proved. It is shown that any theory of gravitation can always be formulated in terms of Minkowski space locally (and globally if spacetime can be covered by a single chart).