  By studying a nonequilibrium Langevin system, we find that a simple condition
determines the decomposition of the coarse-grained force into a dissipative
force, an effective driving force and noise. From this condition, we derive a
new universal inequality, $D \ge \gamma \mud^2 T$, relating the diffusion
constant $D$, the differential mobility $\mud$, the bare friction constant
$\gamma$ and the temperature $T$. Due to the general nature of the argument we
present, we believe that our idea concerning this decomposition can be applied
to a wide class of systems far from equilibrium.
