  The problem of nonuniqueness of minimal coupling procedure for
Einstein--Cartan (EC) gravity with matter is investigated. It is shown that the
predictions of the theory of gravity with fermionic matter can radically change
if the freedom of addition of a divergence to the flat space matter Lagrangean
density is exploited. The well--known gravity induced four--fermion interaction
is shown to reveal unexpected features. The solution to the problem of
nonuniqueness of minimal coupling of EC gravity is argued to be necessary in
order for the theory to produce definite predictions. In particular, the EC
theory with fermions is shown to be indistinguishable from usual General
Relativity on the effective level, if the flat space fermionic Lagrangean is
appropriately chosen. Hence, the solution to the problem of nonuniqueness of
minimal coupling procedure is argued to be necessary if EC theory is to be
experimentally verifiable. It could also enable experimental tests of theories
based on EC, such as loop approach to quantisation of gravitational field. Some
ideas of how the arbitrariness incorporated in EC theory could be restricted or
even eliminated are presented.
