  In a recent work we have found a contraction semigroup able to correctly
approximate a projected and perturbed one-parameter group of isometries in a
generic Banach space, in the limit of weak-coupling. Here we study its
generator by specializing to $W^*$-algebras: after defining a Physical
Subsystem in terms of a completely positive projecting conditional expectation,
we find that it generates a Quantum Dynamical Semigroup. As a consequence of
uniqueness and strong generality (well defined dynamics, irrespective of the
Physical Subsystem spectral properties or dimensions), its generator deserves
to be referred as "the" Quantum Fokker-Planck Equation. We then provide
important examples of the limit dynamics, one of which constitutes a new
Quantum generalization of the celebrated Fermi Golden Rule.
