  Almost all presentations of Dirac theory in first or second quantization in
Physics (and Mathematics) textbooks make use of covariant Dirac spinor fields.
An exception is the presentation of that theory (first quantization) offered
originally by Hestenes and now used by many authors. There, a new concept of
spinor field (as a sum of non homogeneous even multivectors fields) is used.
However, a carefully analysis (detailed below) shows that the original Hestenes
definition cannot be correct since it conflicts with the meaning of the Fierz
identities. In this paper we start a program dedicated to the examination of
the mathematical and physical basis for a comprehensive definition of the
objects used by Hestenes. In order to do that we give a preliminary definition
of algebraic spinor fields (ASF) and Dirac-Hestenes spinor fields (DHSF) on
Minkowski spacetime as some equivalence classes of well defined pairs of
mathematical objects, one of the members of the pair being an even
nonhomegeneous differential form.
  The necessity of our definitions are shown by a carefull analysis of possible
formulations of Dirac theory and the meaning of the set of Fierz identities.
  We believe that the present paper clarifies some misunderstandings
  (past and recent) appearing on the literature of the subject. It will be
followed by a sequel paper where definitive definitions of ASF and DHSF are
given as appropriate sections a vector bundle called the left spin-Clifford
bundle. The present paper contains also Appendices (A-E) which exhibits a truly
useful collection of results concerning the theory of Clifford algebras
(including many `tricks of the trade') necessary for the intelligibility of the
text.
