  It is shown that the Dirac theory implies complex space-time and complex
space-time can lead to the Dirac equation. It is suggested that fermions are
grouped into doublets, those doublets are then divided into color singlets
(leptons) and color triplets (quarks), then they are further divided into
generation singlets and generation triplets. If the exclusion principle for
fermions works in the internal space, then the non-observation of free quarks
can be explained. However, it is suggested that a possible new free quark may
exist with a color triplet and generation singlet state which is antisymmetric
in the internal space.
