It is shown that models of elementary particles in classical general relati
vity (geons) will naturally have the transformation properties of a spinor
if the spacetime manifold is not time orientable. From a purely pragmatic i
nterpretation of quantum theory this explains why spinor fields are needed
to represent particles. The models are based entirely on classical general
relativity and are motivated by the suggestion that the lack of a time-orie
ntation could be the origin of quantum phenomena.