  When one tries to fit Dirac’s theory of the electron into general  relativity, it becomes imperative to adopt the Cartan method. For  Dirac’s four \(\psi\)-components are relative to a Cartesian (or rather  a Lorentz) frame. One knows how they transform under transition from  one Lorentz frame to another (spin representation of the Lorentz  group); but this law of transformation is of such a nature that it  cannot be extended to arbitrary linear transformations mediating  between affine frames.