(2.3.4) Reining : I agree, in principle, that we should get from the density all properties that are determined by the external potential and the number of electrons. Why do we then feel that we have so little diversity in the observables that are traditionally dealt with in DFT? First, this statement is actually not true, if we consider the Kohn-Sham observables as approximations to the true density functional of, for example, spectra -there are many such calculations around. The Kohn-Sham expressions are of course not explicit functionals of the density, but implicit ones, via the orbitals. But why is it so difficult to go beyond the Kohn-Sham approximation and find better ones for these observables?