  The master equation for a linear open quantum system in a general environment
is derived using a stochastic approach. This is an alternative derivation to
that of Hu, Paz and Zhang, which was based on the direct computation of path
integrals, or to that of Halliwell and Yu, based on the evolution of the Wigner
function for a linear closed quantum system. We first show by using the
influence functinal formalism that the reduced Wigner function for the open
system coincides with a distribution function resulting from averaging both
over the initial conditions and the stochastic source of a formal Langevin
equation. The master equation for the reduced Wigner function can then be
deduced as a Fokker-Planck equation obtained from the formal Langevin equation.
