  Many sets of polarization evolution equations have been suggested to describe
the method of polarization buildup by spin filtering in storage rings. In this
paper a generic system of polarization evolution equations describing spin
filtering is derived and solved, then we compare and contrast this system to
other descriptions of spin filtering appearing in the literature. This is of
interest to projects planning to produce a polarized antiproton beam by spin
filtering, and to any project utilizing spin filtering in storage rings. The
physical processes responsible for spin filtering are analysed and their
contributions to the dynamics of polarization buildup are highlighted. It is
hoped that this will shed light on some of the confusion in the literature.
