I assume that everyone knows the constitutive relation of dielectric material : D = ε_0E + P where D is the electric field displacement ε_0 is permittivity of vacumm E is the electric field (or may be the electric ) and P is the polarization (density*dipole) I am confusing about the electric field E. Is it the external electric field or the local electric field in the dielectric material. I think that it is the local electric field in the dielectric material. If we apply the external electric field E0 to this material, the local electric field should be changed due to the induced electric field. However, most of classical theories of metal and dielectric materials use E0 in the constitutive relation. It is obviously incorrect. What do you think?