In theoretical physics, a source field is a field
appears in the action, multiplied by the original field
All Green's functions (correlators) may be formally found via Taylor expansion of the partition sum considered as a function of the source fields. This method is commonly used in the path integral formulation of quantum field theory. The general method by which such source fields can be utilized to obtain propagators in both quantum, statistical-mechanics and other systems is outlined in the article on the partition function.