A formal solution for Green’s functions of the type [∂2+m2+gV (x)]G (x,y) =δ (x−y) is presented which has the structure of an effective mass formalism. One first solves the free case [gV (x) =0] for given boundary conditions and then replaces the parameter m in the solution by a quantity depending on V (x). The rules for this replacement are given, a connection with the Baker–Campbell–Hausdorff formula is established, and it is shown how the formalism unites different perturbation and approximation schemes.