  A method is presented to obtain the linear response coefficients of a system
coupled to a bath. The method is based on a systematic truncation of the
Liouville equation for the reduced distribution function. The first order
truncation results are expected to be accurate in the low temperature and weak
coupling regime. Explicit expressions for the conductivity of the Fr\"ohlich
polaron are obtained, and the discrepancy between the Kadanoff and the
Feynman-Hellwarth-Iddings-Platzmann mobility is elucidated.
