  We show how the linearized equations of motion of any dissipative current are
determined by the analytical structure of the associated retarded Green's
function. If the singularity of the Green's function, which is nearest to the
origin in the complex-frequency plane, is a simple pole on the imaginary
frequency axis, the linearized equations of motion can be reduced to
relaxation-type equations for the dissipative currents. The value of the
relaxation time is given by the inverse of this pole. We prove that, if the
relaxation time is sent to zero, or equivalently, the pole to infinity, the
dissipative currents approach the values given by the standard gradient
expansion.
