So, in a book on QFT there is in the begining some talk about Klein-Gordon field and equation. This is solved by using simple harmonic oscilator formalism and a spectrum for a free H is found. But then, they put a general space dependent function term on the right hand side of the KG equation making it inhomogenius and call it a source term. But we had a field even before this source term. So a few questions come to mind. First, what is a source of any quantum field? It seems that it does not have one. It just is. Second, if this source term is not the source of the field but just some term to put some interaction, what is the analogy that is used to justify that idea? If we look at first field that was quantized, EM field, clasicaly we know that in order to have a electromagnetic field we need some charged bodies. Again, we come to my first question. In qft, is it correct to say, that EM field which is quantized to give photons, just IS and does not need any sources, and that charged particles interact with the field itself to excite it and give rise to photons? So, if we have to charges then we can say that they do not create the field but they interact with it? SO, in the KG case, when we put that source term, do we just put something of the sort of a "charge" which is actually coupled with the field in a similar way as electric charge is coupled to the EM field?