What is more logical than continuing along this line and taking out another part (the kinetic energy of some noninteracting system)? And what is more logical than taking this noninteracting system to be "similar" to the real system -with the same density, in the spirit of DFT? Generalized Kohn-Sham theory is then also very natural, both because we know more pieces and because (like the kinetic energy) we do not know them as explicit density functionals. Making these pieces and the resulting "potentials" more and more complex appears to build a continuous bridge between Kohn-Sham and Green's functions equations. Another generalization is to start with the consideration that the calculation of any observable will in general integrate out certain details of a system, so the same value for the observable might well be found in a simpler system. This holds for the density -with the Kohn-Sham system, for example -but one can also build auxiliary systems for other observables and profit from the Kohn-Sham experience.