  An equation for the reduced density matrix which describes a free particle,
that is interacting with a linearly dissipative medium, is derived using the
total Hamiltonian, and without resorting to any artificial model.
  A Master equation is also obtained for the dissipative medium. The theory
erases the notion of the reservoir. It is shown that the dynamical interaction
with the medium is not completely determined by the friction constant (or the
corresponding fluctuation-dissipation relation). Unexpectedly, through the
density-density correlation, the longitudinal dielectric function is shown to
play a critical role.
