  Although intrinsic spin is usually viewed as a purely quantum property with
no classical analog, we present evidence here that fermion spin has a classical
origin rooted in the geometry of three-dimensional physical space. Our approach
to the quantum/classical interface is based on a formulation of relativistic
classical mechanics that uses spinors. Spinors and projectors arise naturally
in the Clifford's geometric algebra of physical space and not only provide
powerful tools for solving problems in classical electrodynamics, but also
reproduce a number of quantum results. In particular, many properites of
elementary fermions, as spin-1/2 particles, are obtained classically and relate
spin, the associated g-factor, its coupling to an external magnetic field, and
its magnetic moment to Zitterbewegung and de Broglie waves. The relationship is
further strengthened by the fact that physical space and its geometric algebra
can be derived from fermion annihilation and creation operators. The approach
resolves Pauli's argument against treating time as an operator by recognizing
phase factors as projected rotation operators.
