I just picked up a book on QFT and have already found myself stuck on something that appears to be quite elementary. When disturbing the field, the potential gets an added term with a source function [itex]J[/itex], [tex]J(x, t) \phi(x).[/tex] I'm not quite getting what [itex]J[/itex] physically represents. It 'describes how the field is being disturbed', but how exactly? Also, why is the added term necessarily linear in [itex]\phi[/itex]?