  On the basis of the Kubo-Luttinger linear response theory combined with the
scaling theory of Anderson localization predicting the energy dependence of
localization length near the mobility edge, we have studied the thermoelectric
response of a disordered system exhibiting Mott variable-range hopping (VRH).
We found that the low-tempetature Seebeck coefficient for VRH conduction in a
$d$-dimensional system varies as $S\propto T^{d/(d+1)}$, which is different
from the widely used expression, $S\propto T^{(d-1)/(d+1)}$, based on the
energy-independent localization length. It is seen that the low-$T$ behavior of
$S(T)$ of the thiospinel CuCrTiS$_4$ is in full agreement with the new scheme.
