  We present a succinct and intuitive derivation of a formally exact master
equation for general open quantum systems, without the use of an "inverse" map
which was invoked in previous works on formally exact master equations. This
formalism is applicable to non-Markovian regimes. We derive a second-order
equation of motion for the illustrative spin-boson model at arbitrary
temperatures, observing non-exponential decoherence and relaxation. Limiting
our generic derivation to zero temperature, we also reproduce the result for
the special case of a vacuum bath in Phys. Rev. A 81, 042103 (2010).
