When a system in thermal equilibrium is exposed to arbitrary but weak electromagnetic fields, currents are induced by the fields, proportional to the field strengths. The Kubo formalism allows a general discussion of this “linear response”, and the calculation has been carried out by Nakajima. However, the separation of the induced current into an “electric” and a “magnetization” current requires additional physical assumptions. This separation is carried out here. The “electric” current is then further separated into in phase and out of phase contributions. This yields an explicit formula for the complex dielectric constant as a function of the wave number and frequency of the impressed electric field. These formulas have the same generality as the formulas of equilibrium statistical mechanics.