  The present contribution is based on the assumption that the probabilistic
character of quantum mechanics does not originate from uncertainties caused by
the process of measurement or observation, but rather reflects the presence of
objectively existing vacuum fluctuations whose action on massive particles is
calibrated by Planck's constant and effects an additional irregular motion. We
describe a one-particle system by introducing two equally large sub-ensembles
for each of which one obtains an equation of motion that has the form of a
Navier-Stokes- or ''anti''-Navier-Stokes-type equation, respectively. By
averaging over the total ensemble one obtains a new equation of motion which
can be converted into the time-dependent Schroedinger equation. The concept
allows the inclusion of electromagnetic fields and can be extended to
interacting N-particle systems. We analyze the problem of how an experimental
setup can consistently be decomposed into the quantum system under study and
the residual quantum system ''apparatus''. The irregular extra motion of the
particle under study allows a decomposition of the associated ensemble into two
sub-ensembles the members of which perform, respectively, a right-handed or
left-handed irregular circular motion about a given axis which becomes
physically relevant in the presence of a magnetic field. We demonstrate that
this orientation-decomposed ''Zitterbewegung'' behaves - in accordance with
Schroedinger's original idea - as a spin-type angular momentum which appears in
addition to a possible orbital angular moment of the particle. We derive the
non-relativistic time-dependent Pauli equation and propose a theory of the
Stern-Gerlach experiment. The Dirac equation can be derived similarly.
