Ok - so essentially the Feynman propogator is a Green function solved for a point source - i.e a delta function in 3 dims. My question is more mathematical. When you solve for the Green Function you assume (id-m)G(x,x') = delta(x-x') (roughly) Then solving this going through the standard process (Fourier Transforming RHS, performing integral etc) you come up with the Green function aka the Feynman propogator. Now my question is this: What is the original justification (for ALL Green function proplems actually) that the RHS is a delta function?? Why/How do you assume this and what is the physics meaning??